Disadvantages

Multiple Units do have some disadvantages as compared to locomotive hauled trains:

• Maintenance – It may be easier to maintain one locomotive than many self-propelled cars.

• Safety – In the past it was often safer to locate the train’s power systems away from passengers. This was particularly the case for steam locomotives, but still has some relevance for other power sources. A head on collision or level-crossing accident involving a multiple-unit (with passengers potentially right at the front of the train) is likely to result in more casualties than one with a locomotive (where the heavy locomotive would act as a ‘crumple zone’).

• Easy replacement of motive power – If a locomotive fails, it can be easily replaced with minimal shunting movements. There would be no need for passengers to evacuate the train. Failure of a multiple unit will often require a whole new train and time-consuming switching activities; also passengers would be asked to evacuate the failed train and board another one.

• Efficiency – Idle trains do not waste expensive motive power resources. Separate locomotives mean that the costly motive power assets can be moved around as needed and also used for hauling freight trains. A multiple – unit arrangement would limit these costly motive power resources for use in passenger transportation.

• Gangways – It is difficult to have gangways between coupled sets, and still retain an aerodynamic leading front end. Because of this fact, frequently there is no passage between high-speed coupled sets. In such instances more crew resources may be required, as, for example, ticket inspectors, should be present in all of them. This leads to higher crew costs and lower utilization of crew resources. In a locomotive – hauled train, one single crew can serve all the train regardless of the number of cars in the train unless limits of individual workload are not exceeded. Likewise, in such instances, buffet cars and other shared passenger facilities may need to be duplicated in each train set, reducing efficiency.

• Flexibility – Large locomotives can be substituted for small locomotives when more power is needed. Also, different types of passenger cars (such as reclining-seats, compartment cars, couchettes, sleepers, restaurant cars, buffet cars etc.) can be easily added to or removed from a locomotive-hauled train. However, it is not so easy for a multiple unit since individual cars can be attached or detached only in a maintenance facility. This also allows a loco – hauled train to be flexible in terms of number of cars. Cars can be removed or added one by one, but in multiple units two or more units have to be coupled. This is not so flexible.

• Noise and vibration – The passenger environment of a multiple unit is often noticeably noisier than that of a locomotive-hauled train, due to the presence of underfloor machinery. The same applies to vibration. This is a particular problem with DMUs.

• Obsolescence cycles – Separating the motive power from the payload-carrying cars means that either can be replaced when obsolete without affecting the other.