Different realisations of articulated trains exist:

• Jakob-type bogies: Consecutive cars rest on a shared two-axle bogie.
• Single-axle bogies of the KERF type
• Talgo trains: consecutive cars rest on a single axle running gear located between the two cars.

Figure 1 gives an overview.

The following evaluation refers to Jakob-type bogies being the most common running gear for articulated trains. However, many statements are true for articulated trains in general.

Figure 1: Different realisations of articulated trains

Source: IZT

Train design and passenger comfort

Due to less train length car-bodies of articulated trains can be 10-20 cm wider than those of conventional trains for the same track and tunnel profiles. This improves seating comfort in 2 2 second class seating and allows to extend 2 2 seating (rather than 2  1 seating) to first class sections. Since car ends rest on joint bogies they do not sway out in curves. Due to this fact, interior car transitions can be designed to be wider than in conventional stock.

Weight reduction

Besides reduced energy consumption, weight reduction leads to higher acceleration rates for given traction power which is especially relevant in high-speed service.

Infrastructure

Since articulated trains form fixed car sets and cannot be easily decoupled into single cars, they put special requirements on the operator especially when it comes to maintenance and repairs. For most articulated trains very long maintenance workshops are needed (typically 150 m). This is one of the main reasons, why countries like Germany and Switzerland are reluctant to introduce main-line trains with Jakob-type bogies. However, depending on the existing infrastructure of the operator and the actual length of the train sets, the introduction does not always have to create major transition costs for the operator (cf. General criteria - Example).

Flexibility of train composition

In addition, the fixed train composition leads to less flexibility in train length. However, conventional MUs have the same restriction since traction components are distributed along the train-set and therefore the train cannot be decoupled into autonomous units. In both cases, the solution is to achieve flexibility by having short train sets that may be combined to form trains of variable length.

Axle load

Since the total weight is shared by less axles, articulated trains require additional lightweight efforts in order to keep axle-load below 16 tons.

IC3 at DSB

On a great part of their main line passenger service, DSB relies on the IC3, a diesel-mechanic train with Jakob-bogies. Before introducing the articulated train, it was checked if the workshops were prepared, especially as far as lifting of the whole train-set is concerned. Due to relatively short train-sets (59 meters), the transition efforts were limited.

The main energy efficiency effect comes from reduced weight. General figures on the weight reduction effect through Jakob-type bogies are not available, but the following estimate illustrates the potential:

• In a conventional EMU, the mass of the bogies accounts for about one third of the total train weight.
• The introduction of Jakob-type bogies reduces the ratio of bogies to cars by 30-50% (depending on train length).
• On the other hand, the car length is reduced by about 25 %.
• Therefore the number of bogies per train length is reduced by roughly 10-30%.
• Assuming that Jakob-type bogies have roughly the same weight as conventional bogies, this means that the weight of bogies per train length is reduced by 10 - 30%.
• This means a mass reduction potential of about 3 - 10 %, given that bogies account for about one third of the total train weight.

The following elasticity table gives the effect on overall energy demand for different types of operation:

Source: IZT

Maintenance facilities

Depending on the length of the fixed train-sets, maintenance facilities have to be extended. In high-speed transport articulated trains are typically 150 meters long and require corresponding workshops for maintenance and repairs.

The transition costs are lower for the shorter train-sets used in local and regional transport. The Danish IC3 train-sets with a length of 59 m show that even in main-line transport short units can be used (of course in this particular case this possibility is owed to the special IC3 design allowing to fold away the driver cabins in order to create regular car transitions for passengers).