generic framework for transport network designs: applications and treatment in intermodal freight...
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Generic Framework for TransportNetwork Designs: Applications andTreatment in Intermodal FreightTransport LiteratureJohan Woxenius aa Department of Technology Management and Economics,Division of Logistics and Transportation, Chalmers Universityof Technology, Gothenburg, and Department of Systems andSoftware Engineering , Blekinge Institute of Technology ,Karlshamn, SwedenPublished online: 16 Oct 2007.
To cite this article: Johan Woxenius (2007) Generic Framework for Transport Network Designs:Applications and Treatment in Intermodal Freight Transport Literature, Transport Reviews: ATransnational Transdisciplinary Journal, 27:6, 733-749, DOI: 10.1080/01441640701358796
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Transport Reviews, Vol. 27, No. 6, 733–749, November 2007
0144-1647 print/1464-5327 online/07/060733-17 © 2007 Taylor & Francis DOI: 10.1080/01441640701358796
Generic Framework for Transport Network Designs: Applications and Treatment in Intermodal Freight Transport Literature
JOHAN WOXENIUS
Department of Technology Management and Economics, Division of Logistics and Transportation, Chalmers University of Technology, Gothenburg, and Department of Systems and Software Engineering, Blekinge Institute of Technology, Karlshamn, SwedenTaylor and Francis LtdTTRV-A-235771.sgm
(Received 31 October 2006; revised 2 January 2007; accepted 24 January 2007)10.1080/01441640701358796Transport Reviews0144-1647 (print)/1464-5327 (online)Original Article2007Taylor & [email protected]
ABSTRACT Six principles for the design of transport systems are described, includingdirect link, corridor, hub-and-spoke, connected hubs, static routes, and dynamic routes.The designs are theoretically discussed, defining the operational character of each designand their application in passenger, freight and rail freight transport. The theory is thenapplied to intermodal freight transport by comparing the terminology used in the paperwith that in the scientific literature. The advantages of using a generic terminology overcontextual ones are identified from the perspectives of researchers, commercial operatorsand policy-makers.
Introduction
From the perspective of the shipper as the ultimate user of freight transportservices and at the abstraction level of material flows, consignments are generallyseen to move directly from origin to destination. In reality, however, the direct-ness of transport services depends on the economic and practical viability ofconsolidation, defined by Bookbinder and Higginson (2002, p. 305) as “an activeeffort to more efficiently utilise transportation resources”. The phenomenon isalso referred to as bundling, simply defined by Macharis et al. (2002, p. 1) as a“collection of goods to fill a transport unit”. Also, mode-specific terms denoteconsolidation activities, primarily in rail freight with shunting and marshalling,or the terms ‘classification’, ‘grouping’, and ‘blocking’ (Assad, 1980) morefrequently used in the USA. The decision whether to consolidate depends on anumber of parameters:
● Consignment size: the closer to full capacity of a transport, the more direct.● Transport distance: the shorter, the more direct.
Correspondence Address: Johan Woxenius, Department of Technology Management and Economics,Division of Logistics and Transportation, Chalmers University of Technology, SE-412 96 Gothenburg,Sweden. Email: [email protected]
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● Transport time demand: the more specific, the more direct. ● Product characteristics: the more specific, the more direct.● Availability of other goods along the route: the less the availability, the more
direct.
If consolidating flows is decided upon, it is generally done systematically, i.e.according to a transport network design. Most textbooks in the field of transportinclude denotation of such network designs. The terminology, however, is farfrom unanimous between authors and it often varies depending on the context ofgeography and traffic mode. Moreover, despite the significance of basic terminol-ogy, most academic papers addressing routing and consolidation fail to define theoptions clearly. Cases when the same terms are used for denoting differentoperational practices are common, and they easily lead to confusion. Thisobstructs communication among researchers, among practitioners as well asbetween the two groups.
The primary purpose of this paper is to suggest a generic framework for consol-idation and routing principles in a transport network. The framework consists ofsix significantly different theoretical designs: direct link, corridor, hub-and-spoke,connected hubs, static routes, and dynamic routes.
The secondary purpose is to investigate how traffic designs are used incommercial practice and in the scientific literature. Scrutinizing this requires anarrower focus and that chosen focus is the rail part of intermodal road–railfreight transport. The reason for choosing intermodal transport is that it operateson a large scale, relying on the consolidation of unit loads into trains. It alsoreaches across traffic mode borders and involves several categories of actors. Theincentives for developing and using a generic terminology are consequentlyhigher than that for the individual traffic modes. Intermodal transport is also anintegral part of transport policy in Europe (European Commission, 2001, 2006),Japan (Saito et al., 2004) and prospectively making its way into US transportationpolicies (Brown and Hatch, 2002). Legislation clearly requires firm terminology soif a coherent parlance exists, the chances are good that it will be found in the fieldof intermodal transport. The choice is also motivated by the fact that thiscombination of modes has received abundant attention from researchers, henceallowing for an analysis of how the terminology is scientifically used.
The first part of the paper focuses on the framework of transport networkdesigns. They are first presented graphically and briefly described in a genericsetting. They are then described in more depth, including operational character-istics, and illustrated with applications in passenger and freight transportservices. The second part begins with a discussion on how intermodal operatorsuse the options for transport network operation. The following analysiscompares how rail network operation models are denoted in the scientific litera-ture with the generic terminology suggested in this paper. Conclusions andimplications are drawn from the perspectives of researchers, commercial actorsand policy-makers.
Transport Network Designs
This section presents six transport network designs suggested for further use inthe transport sector. The aim is not to prove fully that these are the only conceiv-able ways of consolidation. It might even be claimed that some of the designs are
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Generic Framework for Transport Network Designs 735
superfluous or strictly theoretical and that the application of hybrids of designs iscommon. It is nevertheless asserted that each of the presented design possessesinherent qualities and matches different preconditions in terms of geography,demography, supply of infrastructure and character of the transport demand. Thechoice of network design is also affected when correct information about theactual demand is captured (Tjokroamidjojo et al., 2006), i.e. if there is support forcentralized decision-making, as investigated by Newman and Yano (2000a).
The terminology suggested here is kept on a level between abstract andapplied. An abstract terminology would consistently use nodes and links. Anapplied framework might be too focused on the offered services, that is theshippers’ perception of the transport operations applying the network designs.The applied terminology is also subject to constant change at the cost of clarity.The advantage of using a compromise is that the theoretical rigour is regarded asacceptable, while familiarity is ensured for both researchers and practitioners.
Figure 1 takes the perspective of a transport system operator. A fixed examplewith ten nodes illustrates the different links used for a transport assignment fromthe origin (O) to the destination (D). The theory is based on the assumption that asufficient supply of infrastructure enables direct links between all terminals in thenetwork and that all terminals are capable of serving as origins and destinationsas well as transfer points. The network operator can decide whether to operate thelinks and nodes itself or use services provided by other operators.Figure 1 Six options for transport from an origin (O) to a destination (D) in a network of ten nodes. Dotted lines show operationally related links in the network designs. In ‘Dynamic routes’, two alternative routes are shown; in all other designs, the routing is predefinedIn the direct link alternative, transport is obviously direct from O to D, andthere is no coordination with transport between other O-D pairs. Also, no othernodes are involved.
The transport corridor is a design based on using a high-density flow along anartery and short capillary services to nodes off the corridor. The nodes are thushierarchically ordered. In this example, O is a satellite node and D is a corridor node.
In the hub-and-spoke layout, one node is designated the hub, and all transportscall this node for transfer, even for transports between adjacent origins anddestinations. While the operations follow simple principles, the challenge is tocoordinate a large number of interdependent transport services.
The connected hubs design is another hierarchical layout in which local flowsare collected at hubs that in turn are connected to other hubs in other regions. Itcan thus be described as a direct link with regional consolidation.
Corridor
O
D
Hub-and-spoke
O
D
Static routes
O
D
Direct link
O
D
Dynamic routes
12
O
D
12
21
Connected hubs
O
D
Figure 1 Six options for transport from an origin (O) to a destination (D) in a network of ten nodes. Dotted lines show operationally related links in the network designs. In ‘Dynamic routes’, two
alternative routes are shown; in all other designs, the routing is predefined
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When using the static routes design, the transport operator designates anumber of links to use on a regular basis. In contrast to the hub-and-spoke layout,several nodes are used as transfer points along the route. Transfer is not needed atevery node. Usually, only a part of the load is transferred, and the rest stays onthe transport means to the next node. In Figure 1, O is on a one-way loop,connected by a feeder link to a two-way loop, which in turn is connected to Dthrough another node.
Maximum flexibility is offered by the dynamic routes design. Links aredesignated depending on actual demand, and the network operator can choosemany different routes between O and D. Transport services are planned byheuristics or optimization methods. In an extreme form, routes can be changedduring transportation.
Operational Characteristics and Applications of the Network Designs
The network designs are further explained in this section. The focus is onoperational advantages and disadvantages and the usual context of each design.Real-world applications in services for passengers, freight and rail freight are alsopresented.
The simple direct link design facilitates the highest degree of flexibility and isobviously the most efficient, provided that there is a sufficiently large flow for therequired frequency. The timing is independent of other transports, except whenconsidering congestion and the timing of the return transport. The smaller vehiclesize for road transport compared with sea and rail transport implies a highershare of direct transport, which explains much of the success of the road trafficmode.
Passenger services are exemplified by taxis, while freight services are exempli-fied by full truckloads. There is a certain element of convenience involved, since adirect link service is often used, although an assessment strictly based on cost andtransport quality would recommend using a consolidation service (Sommar,2006). Rail freight requires more planning and consideration (Assad, 1980), whichmight foster more rational decisions. Here, the direct link design is commonlyused for moving large quantities of commodities, but is used for technicallyspecialized services as well.
Corridors often stem from old concentrations of population and industry inlinear belts with mineral resources or fertile soil. Another origin is the supply ofinfrastructures, particularly rivers and canals, but also older roads and rail trunklines that over time have fostered the emergence of conurbations along the line(Priemus and Zonneveld, 2003), sometimes going back to antiquity, as describedby Schönharting et al. (2003). The traffic modes are often poorly integrated alongcorridors (Priemus and Zonneveld, 2003) but Rodrigue (2004) identifies that thefragmentation stemming from intramodal competition is being reduced. Instead,the modes act complementary along corridors while avoiding congestion as aprimary driving force.
Examples of mega-corridors are found along the Japanese east coast, the Bost-Wash corridor in the USA (Rodrigue, 2004), the Rhein–Ruhr–Randstadt/Flemishdiamond corridor in Western Europe (Schönharting et al., 2003), and the WestMidlands to London corridor in the UK, which is defined by Chapman et al.(2003) as a complex area of ‘braided’ infrastructure. The opportunities foreconomic development along the corridor often lead to unregulated urbanization
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Generic Framework for Transport Network Designs 737
and congestion. This has lead to some reluctance actively to develop corridors (DeVries and Priemus, 2003) and sometimes even hostilities on the part of spatialplanners (Priemus and Zonneveld, 2003).
Operationally, the corridor design requires short stops at intermediate termi-nals in order to keep a reasonable transport time along the full corridor. If thecapillary traffic is late to the corridor terminals, it will probably have to wait forthe next departure. A high frequency and slack capacity limits the consequences.A specific challenge for operators is to weigh the slack capacity against therevenues from a high filling grade.
A typical application of the corridor design is the intercity passenger trainswith frequent stops along the line. Particularly, corridors offering high-speedservices attract flows from the areas next to the corridor, provided that the trainsstop at intermediate stations. For geographical reasons, freight traffic with bargeson inland waterways, as investigated by Wiegmans (2005), utilizes the corridordesign, while the US structure with Class I railroads, cooperating with feedershort lines represents a rail freight application.
The major benefit of hub-and-spoke is the ability to connect a large number oforigins and destinations with a high frequency, although the flow between eachO-D pair is small. In a pure hub-and-spoke system, only two links are needed toconnect all origins and destinations.
Hub-and-spoke systems evolve in areas with a dominating centre and depen-dent satellites. In Europe, France is an archetype of a hub-and-spoke system withParis having a dominating role in French society. All French high-speed passen-ger trains, for instance, serve Paris. Many features of a hub-and-spoke system canalso be distinguished in the UK with London as obvious hub. Yet, most hub-and-spoke systems have been actively created by transport operators. The hub offreight systems is generally coarsely localized after calculating the minimal totaltransport cost; while the passenger hub is selected among centrally localizedterminals with a significant role as origin and destination. The structure of hub-and-spoke systems has attracted significant attention from researchers. Forexample, Horner and O’Kelly (2001) use a non-linear cost function to analyse theattractiveness of different hub locations; and Groothedde et al. (2005) show how ahub network collaboratively used by several networks can increase efficiency andassist in making intermodal transport competitive against road transport.
The basic idea behind hub-and-spoke systems is that a high filling grade in thetransport means compensates for the longer transport distances and the transfer.High resource utilization can be accomplished if there are no transport timerestrictions, as goods and transport means can be held at all terminals until itcorresponds with the capacity of the transport means. Most applications,however, adhere to a strict schedule where traffic on all spokes join at the hub fora quick transfer of a large number of items. This operation obviously requires arational hub (Hesse and Rodrigue, 2004).
The typical applications for this design are found in air transportation, astreated in detail by O’Kelly (1998). US domestic airlines operate hub-and-spokesystems, and for intra-European flights, the history of cabotage regulationexplains much of the hub-and-spoke structure. There are few European examplesin rail freight, although the German intermodal operators Transfracht™ andBoxXpress™ operate hub-and-spoke systems (Gouvernal and Daydou, 2005).Also, Intercontainer-Interfrigo™ operates a hub system for volumes that do notallow direct trains (Intercontainer-Interfrigo, 2006). The US double-stack trains for
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maritime containers largely operate through Chicago, Illinois, although a singleterminal is not yet available, as investigated by Rodrigue (2006). With anestimated 14 million 20-foot equivalent units (TEUs) in 2004 (Rawling, 2006), thefreight concentration in Chicago actually compares with the major maritimecontainer ports.
On a local scale, the connected hubs design resembles a hub-and-spoke system,with some authors (e.g. O’Kelly, 1998; Horner and O’Kelly, 2001; Racunica andWynter, 2005) including it in the hub-and-spoke definition. This, however, leadsto a blunt terminology, since most forms of consolidation systems would thenalso qualify as hub-and-spoke systems. Equally important is that the separatedefinition of connected hubs does not stipulate the use of spokes to the hubs.Hence, networks operating the collection of hubs in so-called milk runs (forinstance, road-based general cargo systems as described by Sommar, 2006)qualify, although Liu et al. (2003) specifically denote this design as ‘hub-and-spoke with milk runs’. The operational basis is to use comparatively simpleconsolidation to realize the advantages of a direct link.
Except for domestic general cargo by road, a connected hubs pattern generallyappears in international transportation. Passengers travelling by air betweenminor cities in two European nations would probably use three flights throughtwo major hubs. Connected hubs are also used in container shipping wherefeeder ships, barges, rail shuttles (i.e. trains with a fixed number of wagons oper-ating between two terminals) or a large number of trucks feed the hubs withcontainers. Likewise, international wagonload rail freight is commonly operatedwith full trains between two major marshalling yards, such as Hallsberg inSweden and Maschen in Germany.
The static routes design is generally applied jointly by several users of more orless publicly available transport services along predefined lines or in routesfollowing strict schedules. The users can then combine different services toconnect a very large number of origins and destinations. A classic example of thisis urban public transport. Examples of centrally planned and commercially closedsystems are postal services and some general cargo truck services. Rail freightexamples are rare, but there are some resemblances with the production of classicgeneral cargo services, including when the railways offer transport of consign-ments smaller than a full wagon. However, most of these are now discontinued.
Often, the subservices are organized in loops, but the loops do not needcommon nodes for transfer; they can be organized as pick-up and delivery areaswith connecting links. When a transport means is filled, the routes can be short-circuited at any point and an additional unit must back it up. The users’ planningis limited to finding the relevant transfer terminals in case of high frequency andslack capacity. Lacking that, users might need to book capacity in advance, aswell as gain assistance using timetables. Decision support systems, such as inter-net travel planners offered by many urban transport operators, are needed byusers of complex systems. The load plan is crucial for freight services as the load-ing of the transport means must enable handling goods at all nodes.
The dynamic routes design differs from static routes in the sense that it isgenerally carried out differently each time. The difference refers either to the pathtaken through the network or to the timing of departures and arrivals at termi-nals. Timely capture and processing of demand data are obviously crucial for theplanning process. Full information in advance is the common assumption foroptimization exercises, but a rare experience for transport planners. Here,
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Generic Framework for Transport Network Designs 739
Tjokroamidjojo et al. (2006) have contributed their theory on what happens whenboth consignments and demand data become available over time.
A similar situation to the dynamic routes design is defined by Horner andO’Kelly (2001, p. 255) who denote “a hybrid approach to hub-and-spoke networkdesign” exemplified by a central planner of a bus system and a less-than-truck-load service. Passengers, however, rarely accept dynamic routing. Some passen-gers might change their plans during travel, causing a significant informationproblem. This relates both to capturing travel demand data and to disseminatinginformation about the coming schedule. A common passenger service based ondynamic routes, however, is an airport limousine service. Operating on a compar-atively small scale, the driver simply asks the passengers which hotels they wantto access and adjusts the route accordingly.
For larger flows, dynamic routes are best applied in centrally planned systems.The archetype of freight applications is part load trucking, i.e. less-than-truckloadoperating without terminals where all consignments stay on the truck. Transportplanners then process demand data from the booking system, either by rules ofthumb or assisted by optimization tools. Old wagonload systems, with severalmarshalling yards and several optional routes through the system, were based ondynamic routes. Without strict time demands, wagons could be assigned to trainsgoing approximately in the right direction and successively approach the finaldestination.
The examples of applications found in passenger and freight transportation, aswell as in rail freight transport, are summarized in Table 1.
Transport networks can have an utterly complex design, and the layout princi-ples are, of course, not mutually exclusive. The example of domestic hub-and-spoke systems, in combination with other domestic systems making up aconnected hubs system, has already been mentioned. If the hubs themselves aresignificant origins and destinations, users of a direct link are then combined withusers of a connected hubs design. Hence, users and operators can perceivenetworks differently. A passenger or travel agent might perceive most passengerservices as static routes, while individual transport operators define their servicesas any of the other designs, except dynamic routes.
It is also conceivable to combine direct links with a hub-and-spoke system. Liuet al. (2003) estimate a potential savings in total distance as 10% when using this
Table 1 Typical applications of the different transport network designs in transport services
Direct link CorridorHub-and-spoke Connected hubs Static routes
Dynamic routes
Passenger taxi service intercity train service
domestic airline traffic
intercontinental airline traffic
urban public transport systems
airport limousine service
Freight full truckload service
transport on inland waterways
air transport of express cargo
container shipping
mail and general cargo truck service
part load truck service
Rail freight
specialized large-scale solutions
US Class 1 railroads and short-lines
US double-stack of maritime containers
international wagonload traffic
classic general cargo service
old wagonload with frequent shunting operation
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system compared with operating according to one of the other designs. Also, asystem for very large flows can be improved by superposing the direct link, sincetransport means are rarely exactly full between all origins and destinations. If thesurplus volume is small, it can be forwarded in a consolidation design.
Rail Network Designs in Intermodal Practice
Here, the focus is narrowed to intermodal road–rail freight transport (IRRFT).This section investigates how opportunities of different network designs areutilized in commercial practice. Contextually, focus is placed on the situation inthe European Union and partly in the USA (for more extreme cases, see dedicatedpapers, such as Islam et al. (2006) who investigate the development inBangladesh). In the application of intermodal transport, the dots in Figure 1represent transhipment terminals. Additional pre- and post-haulage by road isneeded at both ends, and the discussion on traffic solutions is limited to the rail-based part of the IRRFT chain. The term ‘intermodal transport’ is used consis-tently with the definitions presented by Janic and Reggiani (2001), which in turnare mainly based on the definitions suggested by the European Conference ofMinisters of Transport (ECMT, 1998). The key to the definition is that at least twotraffic modes are used while keeping the goods in unit loads.
Many policy-makers strongly believe in IRRFT for solving a multitude ofproblems such as excessive energy and land use, emissions, congestion and trafficsafety related to all-road freight transport. Despite stimulating measures, there isstill a significant challenge for intermodal operators to compete with all-roadtransport, defined by Konings and Kreutzberger (2001) and Trip and Bontekoning(2002) as the need for a quality leap.
The advantages of direct trains are evident when flows are large enough for asatisfying frequency, but operating with other network designs also allows IRRFTto compete for O-D pairs characterized by small volumes or short distances. Ashort distance is generally regarded as shorter than the 500 km often mentionedfor Europe (Rutten, 1998; Van Klink and Van den Berg, 1998; Woxenius, 1998a)and Japan (Saito et al., 2004) and the 500 miles (about 800 km) mentioned for theUSA (Gellman, 1994; Newman and Yano, 2000b). A small volume refers to avolume that is less than economically viable for direct trains. This is admittedly ablunt measure, since economically viable direct trains range from a US double-stack train with 100 wagons (Rodrigue, 2006) and a capacity of several hundredTEUs to a Swedish small-scale shuttle train operating with 20 wagons and a 40TEU capacity.
Although Cardebring et al. (2000), in a survey of European intermodal opera-tors, found a wide range of production arrangements in use, there is evidence forclaiming that IRRFT is conventionally produced. The dominating productionparadigm is night-leaps directly between large-scale transhipment terminalsusing gantry cranes and reach stackers (Bärthel and Woxenius, 2004), althoughIntercontainer-Interfrigo™, Transfracht™ and BoxXpress™ operate hub-and-spoke networks, as mentioned above. Starting with Germany in the 1980s and theNetherlands in the 1990s, European railways have gradually abandoned thewagonload production profile for direct trains (Rutten, 1995; Wenger, 2001).According to Woxenius and Bärthel (in press), the trend of abandoning truenetworks for even more direct trains continues; and Gouvernal and Daydou(2005) find that the use of dedicated trains has increased dramatically over the last
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Generic Framework for Transport Network Designs 741
few years in the UK. Nevertheless, Woodburn (2001) reports an increasingnumber of terminals served by the intermodal operator FreightlinerTM in the late1990s. This is, however, more likely explained by the launch of new shuttle trainsto realize Freightliner’s ambition to increase the number of carried containers by50% from the mid-1990s (Woodburn, 2007) than by the use of consolidation in thenetwork design. Perhaps the most typical surrender of a network is French CNC,with a long history of operating a hub-and-spoke system with Paris as the hub,which now limits its operation of shuttle trains to and from ports under the newcompany name, NavilandTM (Naviland, 2006). The Swedish intermodal marketwas one of the last to face the transition as CargoNetTM changed its timetable toinclude only shuttle trains beginning in January 2006 (CargoNet, 2005). Also,North America has seen a geographical concentration of fewer terminals (Slack,1990; Newman and Yano, 2000b).
The traffic network design is influenced by the geography, supply of infra-structure, character of the transport demand and competition with other trafficmodes. Reasons for the operational conservatism can be sought in an inferiorinnovation by European railways (e.g. Loizides and Tsionas, 2002), and in thefact that freight trains are generally yielding the way for passenger trains duringthe daytime (Racunica and Wynter, 2005). It is acknowledged, however, that it isactually truly demanding to operate complex IRRFT systems (Danielis andMarcucci, 2006). The lack of a strong actor taking intermodal leadership isanother reason frequently mentioned (e.g. Woxenius, 1998a; Bontekoning, 2006).However, placing intermodalism at a higher hierarchical level, that is containershipping with hinterland transport by rail, Panayides (2002) suggests a transac-tion-cost theory as a basis for analysing whether a stronger hierarchy is advanta-geous over the current markets among a multitude of actors offering intermodaltransport services.
Bukold (1994, 1996) identifies a flexibility gap between traditional productionmodels for IRRFT. Shuttle and direct trains benefit from economies of scale butare subject to certain capacity risks. Old production models based on consolida-tion by marshalling single wagons or shunting wagon groups do not depend on astable demand but are too expensive to operate. Bukold argues that new flexiblecorridors and hub-and-spoke production models can achieve economies of scaleat much lower-capacity risk levels.
The conservative attitude of IRRFT operators is also disappointing for research-ers addressing operational aspects of intermodal transport. Researchers believethat IRRFT can compete for less-than-train flows as well as shorter distances.There is a substantial supply of published research on alternative traffic operationprinciples as well as wagon and transhipment technologies (for an overview, seeBontekoning et al., 2004). Inventors have also made significant efforts to developtechnologies facilitating more advanced traffic operations, but very few of theseefforts have been commercially implemented.
Rail Network Designs in the Intermodal Transport Literature
Much of the literature on IRRFT takes the network design for granted and devel-ops methods on how to use that network (e.g. Barnhart and Ratliff, 1993; Nozickand Morlok, 1997; Newman and Yano, 2000a, b, Southworth and Peterson, 2000;Indra-Payoong et al., 2004). Other sets of literature develop methods for locatingterminals, such as Arnold et al. (2004), Tsamboulas (2004), and Racunica and
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Wynter (2005). Meinert et al. (1998) also address terminal location, but their modelallows the simulation of different network designs.
A significant amount of research has also been published on options for design-ing the rail part of IRRFT. Prominent research environments in Europe includeDelft University of Technology (TU Delft) in the Netherlands, the NationalTechnical University of Athens (NTUA) in Greece, Vrije Universiteit Brussel(VUB) in Belgium, Technischen Universität Hamburg-Harburg (TUHH) inGermany, and the Swedish universities, the Royal Institute of Technology (KTH)and Chalmers University of Technology. Some publications from these researchenvironments are listed in Table 2.
Certain contributions have also been made by various constellations in projectswith European Union funding (e.g. Permala et al., 1998; Cardebring et al., 2000).The subject is not exhaustively treated, however, and the industrial trend is, asmentioned, to simplify rail operations rather than apply the advanced trafficdesigns investigated and sometimes normatively proposed by researchers.Despite the differing denotation, most theories on network designs share thesame basic principles. Hence, an attempt is made to compare the terminologyused in this paper with other researchers’ terminology.
Most authors use the term ‘shuttle train’ more narrowly than ‘direct train’, inthe sense that shuttle trains repeatedly operate between two terminals with afixed number of wagons. Specific for rail transport is also the possibility to oper-ate block trains, i.e. groups of wagons that are kept together for consolidationactivities. Oddly, Bukold (1994) and Cardebring et al. (2000) use the term ‘blocktrain’ for denoting a direct link, while the other authors compare a wagon groupsystem and a part train, respectively, to a block train.
At TU Delft the recent works within the TRAIL research school often refer tothe network designs published by Vleugel et al. (2001), where direct begin-endbundling (denoted as direct point-to-point bundling by Trip and Bontekoning,2002) is contrasted with complex bundling. It corresponds well to the direct linkand connected hubs designs used here. Kreutzberger (1999a, b, 2003, 2004) goesfurther and defines five classes of bundling networks. Begin-end networks
Table 2 Examples of publications contributing to the theory on traffic designs in intermodal road–rail freight transport
Research groupExamples of publications addressing rail network designs in intermodal road–rail freight transport
TU Delft Bontekoning and Kreutzberger (1999, 2001), Bontekoning (2006), Bontekoning and Priemus (2004), Konings (1996), Kreutzberger (1999a, b, 2003, 2004), Rutten (1995, 1998), Trip and Bontekoning (2002), Trip and Kreutzberger (2002), Vleugel et al. (2001)
NTUA Ballis and Golias (2002, 2004), Tsamboulas (2004)VUB Macharis and Verbeke (2002), Macharis et al. (2002)TUHH Bukold (1993, 1994, 1996)KTH Nelldal et al. (2000), Nelldal (2005)Chalmers Bärthel and Woxenius (2004), Sommar (2006), Woxenius (1998a, b), Woxenius
and Bärthel (in press), Woxenius et al. (1993)TUHH + Chalmers Woxenius and Sjöstedt (2003)KTH + Chalmers Woxenius et al. (2004)
For abbreviations, see the text.
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Generic Framework for Transport Network Designs 743
correspond to direct link, and hub-and-spoke is specified as one- or all-direc-tional, of which the latter fully corresponds to the hub-and-spoke design used inthis paper. The trunk collection-and-distribution network is similar to connectedhubs, and the line network is similar to corridor. The trunk-feeder network alsoresembles a corridor design, but it differs from a line network by allowing trainsas well as trucks to feed the corridor terminals. Kreutzberger divides both lineand trunk-feeder networks into separated and diffused networks, depending onwhether terminals are specialized in loading or unloading or allow bothoperations at all terminals.
Rutten (1995) uses a similar terminology to that used in this paper. Staticroutes, however, are not directly mentioned but represented by a combination ofother network designs. The difference is explained by different levels of analysis.The terms used by Bukold (1994, 1996) resemble Rutten’s, but they include singlewagon traffic, which is similar to static and dynamic routes.
Nelldal et al. (2000) present seven traffic designs for rail freight, includingconventional wagonload trains. Their term ‘direct train’ refers to long-distancetraffic, which explains why when feeder trains are used, it resembles connectedhubs. In Nelldal (2005) a section is more narrowly devoted to network designs forintermodal transport, and the terminology is slightly different.
The traffic design scheme in this paper is developed from Woxenius et al. (1993)and Woxenius (1998a), where there are obvious similarities. Nevertheless, directlink is denoted as direct connection, and the static routes design is referred to asfixed routes, whereas dynamic routes are referred to as allocated routes and flexi-ble routes, respectively. The connected hubs design was not part of earlier publi-cations. Bärthel and Woxenius (2004) used the term ‘night-haul’ for direct link,referring to the typical timing of direct IRRFT. Accordingly, the static routesmodel is rather similar to short day-hauls, where the trains are used as shortershuttles to close terminals during the day. Woxenius and Bärthel (in press) alsouse hierarchic networks to describe a production profile similar to static anddynamic routes.
Cardebring et al. (2000) define nine intermodal train operating systems. Theframework corresponds rather well to the other authors’ frameworks, except forthe block train definition mentioned above. In addition, they define short lines asshort feeders to major trains, circle trains as a similar operation but in loops, andgateway systems as connections between train operating systems through inter-faces. In combination, they resemble a static routes design. Permala et al. (1998)define five traffic designs for intermodal transport. The terminology correspondspartly to that suggested by Woxenius et al. (1993) but adds hubs and trunk linescorresponding to connected hubs and circulation that can be interpreted as a partof a static route. Table 3 compares the terminology used by other authors to theones used in this paper.
In all, the framework presented in this paper corresponds well to the trafficdesigns presented in the intermodal literature. The more generic terms definedhere are regarded as advantageous for wider communication and for comparingdesigns to applications other than intermodal transport.
Implications and Conclusions
The six routing options in transport networks are the main focus of this paper.The generic framework is suggested for further use in the transport sector, but the
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744 J. Woxenius
Tab
le 3
Com
pari
son
of te
rmin
olog
y re
late
d to
inte
rmod
al tr
ansp
ort n
etw
ork
des
igns
Ref
eren
ces
Dir
ect l
ink
Cor
rid
orH
ub-a
nd-s
poke
Con
nect
ed h
ubs
Stat
ic r
oute
sD
ynam
ic r
oute
s
TU
Del
ft/
TR
AIL
*be
gin–
end
bun
dlin
g,
dir
ect p
oint
-to-
poin
t bu
ndlin
g
line
netw
ork,
trun
k-fe
eder
net
wor
k (s
epar
ated
or
dif
fuse
d)
hub-
and
-spo
ke
(one
dir
ecti
onal
or
all d
irec
tion
al)
trun
k co
llect
ion
and
d
istr
ibut
ion
––
Rut
ten
(199
5)d
irec
t tra
in, s
hutt
le tr
ain
liner
trai
nhu
b-an
d-s
poke
–co
mbi
ning
sev
eral
trai
n op
erat
ion
syst
ems
–
Buk
old
(199
4, 1
996)
bloc
k tr
ain,
shu
ttle
trai
nlin
er tr
ain
hub-
and
-spo
ke–
sing
le w
agon
ssi
ngle
wag
ons
Nel
ldal
(200
5)en
d-p
oint
traf
fic
line
traf
fic
–ju
ncti
on s
yste
map
pr. l
ine
trai
n sy
stem
–N
elld
al e
t al.
(200
0)fu
ll tr
ain,
sys
tem
trai
n,
shut
tle
trai
nlin
e tr
ain
hub
syst
emd
irec
t tra
inap
pr. j
unct
ion
syst
em w
ith
inte
rmed
iate
shu
ntin
g–
Wox
eniu
s et
al.
(199
3),
Wox
eniu
s (1
998a
)d
irec
t con
nect
ion
corr
idor
hub-
and
-spo
ke–
fixe
d r
oute
sal
loca
ted
rou
tes,
fl
exib
le r
oute
sB
ärth
el a
nd W
oxen
ius
(200
4)ni
ght-
haul
cont
inuo
us li
ner
trai
nhu
b-an
d-s
poke
–ap
pr. s
hort
day
-hau
ls–
Wox
eniu
s an
d B
ärth
el
(in
pres
s)d
irec
t con
nect
ion,
sh
uttl
e tr
ain
tran
spor
t cor
rid
orhu
b-an
d-s
poke
–ap
pr. h
iera
rchi
c ne
twor
kap
pr. h
iera
rchi
c ne
twor
kC
ard
ebri
ng e
t al.
(200
0)sh
uttl
e tr
ain,
blo
ck tr
ain
liner
trai
nhu
b sy
stem
appr
. y-s
hutt
le tr
ain
shor
t lin
e, c
ircl
e tr
ain,
ga
tew
ay s
yste
msi
ngle
-wag
on tr
ain
Perm
ala
et a
l. (1
998)
dir
ect c
onne
ctio
nco
rrid
orhu
b-an
d-s
poke
hubs
and
trun
k lin
eap
pr. c
ircu
lati
on–
*Exa
mpl
e in
Bon
teko
ning
and
Kre
utzb
erge
r (1
999,
200
1), B
onte
koni
ng (
1999
, 200
6), K
reut
zber
ger
(200
4), R
utte
n (1
995,
199
8), T
rip
and
Bon
teko
ning
(20
02),
Vle
ugel
et
al.
(200
1).
appr
., A
ppro
xim
atel
y d
enot
ing
the
sam
e th
ing.
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Generic Framework for Transport Network Designs 745
aim is, as mentioned, not to prove fully that these are the only conceivable waysof consolidation or that none of the designs is superfluous. It is neverthelessbelieved that a coherent use of the framework would help a common understand-ing and communication within the sector and facilitate comparisons betweendifferent services. The particular strength is regarded to be the generic characterof the network designs without being too abstract. Thus, it has the potential toreplace some contextual terminology. Some jargon is still needed within trafficmodes and for specific applications, but the continuous replacement of basicterms in industry as well as in academia is often more confusing than helpful.This is well illustrated by the many variants of terminology found in theintermodal freight transport literature. A few of the terms add to understanding,while most research would suffice with more basic and generic terminology.
The implications for transport operators is that there is a toolbox of transportnetwork designs and that there is much to learn from operations designed forother settings. A common terminology might actually be the key to findingobjects for benchmarking. The short review of how different transport designs arecommercially practised is somewhat discouraging for intermodal transportoperators, but there are alternatives to shuttles. There is no natural law prohibit-ing competition with road transport on shorter distances, but consolidation isprobably needed and the network has to be operated wisely.
It is not expected that this paper will lead to a consensus on the use of terminol-ogy for transport network designs among transport researchers; neither is theframework intended to be forced onto someone. There might well be better waysof explaining the phenomenon and this paper will hopefully trigger further workin the field. Still, there is an immediate need for a well-established terminologyfor describing network operation principles.
Implications of this study for intermodal freight transport researchers are thatdespite common projects and publications, the research has not converged andarrived at common definitions or even descriptions of alternative transportnetwork designs. One interpretation of this is that the research field evolves and,thus, so does terminology. Another interpretation is that the object of studydiffers between countries. Yet another interpretation is that the intermodalresearch is fragmented with small groups working on their own problems, similarto what Bontekoning et al. (2004) suggested. Nevertheless, the direct link,corridor, hub-and-spoke and connected hubs network operation principles arecommonly applied in transport systems and exhaustively treated in the scientificliterature, although denoted differently by authors. The static routes and dynamicroutes, however, are studied less often and might well attract further scientificattention.
Finally, policy-makers might need the clearest terminology. This is particularlytrue if transport network design is mentioned in legislation. Nevertheless, a clearterminology is not sufficient; it must also reflect commercial practice and beunderstood by a wide range of actors. The commercial use of different transportnetwork designs in intermodal freight transport is only briefly addressed in thispaper, but the picture is clear: direct links dominate and the use increases at theexpense of consolidation networks. While direct trains offer simple and cost-efficient operations and a very good service on axes with large flows over longdistances, consolidation is a prerequisite for competing with all-road transport onshort distances (Macharis and Verbeke, 2002; Bärthel and Woxenius, 2004;Bontekoning and Priemus, 2004; Bontekoning, 2006). The decline of spatially
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746 J. Woxenius
spread services illustrates a retreat from significant geographical markets. Sincemuch of the transport demand in Europe regards distances below those currentlyserviced by direct trains, focused policy efforts fostering consolidation networksmight be more powerful than general subsidies for intermodal transport.
Acknowledgements
The author would like to acknowledge funding from the Swedish Road and RailAdministrations and the Foundation of the Savings Bank in Karlshamn.
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