Since in MUs the traction components are distributed along the train, the cars of a given set cannot be decoupled. This tends to reduce flexibility of train length. On the other hand, short train-sets can be ordered in order to recover some of the modilarity in train formation typical for loco-hauled train operation.

Need for trains with variable length

Short train-sets offer two main benefits:

• Capacity can be adapted to variable demand (e.g. rush-hour vs. late evening in suburban transport)
• Trains can split up in two train-sets at a certain point of the route to serve two destinations. Passengers do not have to change trains and the operator saves costs.

Realisations

• Short MUs: MUs are ordered as short train-sets (e.g. two-car sets) and can then be combined to double or triple trains for times or routes with high demand. This concept is especially suited to local and suburban service where no passenger mobility along the whole train is needed.
• MUs are ordered in different lengths. For example DB AG ordered the ICE T tilting trains in two lengths: 5-car and 7-car train-sets. Despite higher seat-specific investment costs, this can be interesting in main-line service where passenger mobility along the whole train is required.
• IC3 type: A special case is the Danish IC3 concept (with its characteristic rubber frames at both ends of a train-set). The train sets are short (3-car units) but can be easily coupled to longer trains without limiting passenger mobility along the train. This is achieved by a special design allowing to fold away the driver cabins in order to create regular car transitions for passengers.
• ICE 2 type: The typical configuration of a German ICE 2 consists of two half trains each having a locomotive at one end and a small driving unit (without installed power) at the other end. This allows for a splitting up of the ICE 2 in two half trains in order to serve two routes having the first part of the trip in common (for example the Berlin-Cologne line).

The energy efficiency potential offered by flexible train-sets is difficult to assess in general terms. It depends on spatial and temporal demand variation and train design.

Example

The following (simplified) example may give an idea of the different influencing factors:

On a given line presently operated with 4-car train-sets, 50% of the runs have an average occupancy of 80%, the other 50% only 30%. If 2-car train-sets are introduced instead, 50% of the runs will be realised with two 2-car train-sets coupled together, the rest with only one 2-car set. The energy consumed for all the runs will be referred to as 100%.

A typical situation in suburban transport is assumed with the following components of energy consumption:

• 30% air drag
• 50% acceleration
• 20% comfort functions

Energy balance for times with high occupancy: Using two coupled 2-car sets rather than one 4-car set slightly (< 10%) increases the mass due to more traction equipment. A 10% mass increase will increase energy consumption by about 5%.

Energy balance for the rest of the day: Using a 2-car rather than a 4-car set during hours of low-demand reduces energy demand due to 3 effects:

• Energy demand for comfort functions is reduced by ~50% (=10% of the total consumption)
• With one train set instead of two and 10% higher mass per car (cf. above), mass is reduced by ~45% (=22,5 % of total energy demand)
• Due to reduced train length, air drag is reduced by ~30% (=9% of total energy demand).

Aggregating these data, we get the following energy balance through the use of shorter units:

• Runs during high-demand period consume 5% more energy.
• Runs during low-demand period consume ~41,5 % less energy
• Assuming that 50% of the energy is consumed on 50% of the runs if the same train-sets are used for all runs, one gets a total energy savings of 0,5 ? 41,5% - 0,5 ? 5% = 18%.

Conclusion

The above example simplifies the real situation. It shows however, that an energy saving potential of over 10% is realistic.