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).