Editing Multiple unit (section)

==Comparison to locomotive-hauled trains==
{{original research|section|date=June 2012}}

===Advantages===

====Energy efficiency====
They are more energy-efficient than locomotive-hauled trains. {{citation needed|date=June 2019}}

====Gradients====
They have better [[Adhesion railway|adhesion]], as more of the train's weight is carried on driven wheels, rather than the locomotive having to haul the dead weight of unpowered coaches.

====Acceleration====
They have a higher power-to-weight-ratio than a locomotive-hauled train since they don't have a heavy locomotive that does not itself carry passengers, but contributes to the total weight of the train. This is particularly important where train services make frequent stops, since the energy consumed for accelerating the train increases significantly with an increase in weight. Because of the [[Energy efficiency (physics)|energy efficiency]] and higher adhesive-weight-to-total-weight ratio values, they generally have higher acceleration ability than locomotive-type trains and are favored in urban trains and metro systems for frequent start/stop routines.

====Turnaround times====
Most of them have cabs at both ends, resulting in quicker turnaround times, reduced crewing costs, and enhanced safety. The faster turnaround time and the reduced size (due to higher frequencies) as compared to large [[locomotive]]-hauled trains, has made the MU a major part of [[Commuter rail|suburban]] commuter rail services in many countries. MUs are also used by most rapid transit systems. However, the need to turn a locomotive is no longer a problem for locomotive-hauled trains due to the increasing use of [[Push-pull train|push pull train]]s.

====Failure====
Multiple units may usually be quickly made up or separated into sets of varying lengths. Several multiple units may run as a single train, then be broken at a junction point into shorter trains for different destinations. As there are multiple engines/motors, the failure of one engine does not prevent the train from continuing its journey. A locomotive-drawn train typically has only one power unit, whose failure will disable the train. However, some locomotive-hauled trains may contain more than one power unit and thus be able to continue at reduced speed after the failure of one.

====Axle loads====
They have lighter axle loads, allowing operation on lighter tracks, where locomotives may be banned. Another side effect of this is reduced track wear, as traction forces can be provided through many axles, rather than just the four or six of a locomotive. They generally have rigid couplers instead of the flexible ones often used on locomotive-hauled trains. That means brakes/throttle can be more quickly applied without an excessive amount of jerk experienced in passenger coaches. In a locomotive-hauled train, if the number of cars is changed to meet the demand, acceleration and braking performance will also change. This calls for performance calculations to be done taking the heaviest train composition into account. This may sometimes cause some trains in off-peak periods to be overpowered with respect to the required performance. When 2 or more multiple units are coupled, train performance remains almost unchanged. However, in locomotive-hauled train compositions, using more powerful locomotives when a train is longer can solve this problem.

===Disadvantages===

====Maintenance====
It may be easier to maintain one locomotive than many self-propelled cars. 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 casualties than one with a locomotive{{clarify|date=January 2025}} (where the heavy locomotive would act as a "crumple zone").

====Failure====
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. However, if the train consists of more than one multiple unit they are often designed such that in the event of the failure of one unit others in the train can tow it in neutral if brakes and other safety systems are operational.

====Idle trains====
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 to use in passenger transportation.

====Passage between units====
It is difficult to have [[gangway connection]]s between coupled units and still retain an aerodynamic leading front end. Because of this, there is usually no passage between high-speed coupled units, though lower-speed coupled units frequently have connections between coupled units.{{citation needed|date=November 2017}} This may require more crew members, so that ticket inspectors, for example, can be present in all of them. This leads to higher operating costs and lower use of crew resources. In a locomotive-hauled train, one crew can serve the train regardless of the number of cars in the train provided 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 unit, reducing efficiency.

====Flexibility====
Large locomotives can be used instead of small locomotives where 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-drawn train. This 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 on multiple units two or more units have to be coupled. This is not so flexible.

====Noise====
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 [[Diesel multiple unit|DMU]]s.

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