>>1864346In the context of a commuter rail?
- take up space on stations
- heavy, emus and dmus rarely exceed 12t in axle load, locomotives are usually as heavy as rail operators allow ( 22.5t in eurozone, up to 36t in the US ). Fixed plant maintenance is heavily dependent on the axle load, especially when speeds go up.
- poor acceleration - debatable, but in extreme cases you get less grip because of less powered axles.
- less flexible - two emu's coupled back to back can be quickly split into two smaller trains and run in separate directions, a loco hauled train requires another loco to achieve that.
- varying performance. A train with a loco achieves optimal ( for a given route ) performance in one configuration only. With emus, your 6 section one is, most likely, twice the three section one. With a loco, running 6 cars instead of 3 yields either a train that is grossly overpowered, or one that is grossly underpowered. Moreover - if the train is excessively power that 'excess' power is frozen to that train and cannot be used elsewhere.
It's not to say that locomotives and 'loose' cars don't have their uses, but for a commuter railroad they have a supplementary role. The backbone should always made of MU trains.
PicRel - pathology to the max - a loco hauled commuter train that is semi-permanently coupled because in practice this flexibility is never actually utilized. Newer SBB-CFF-FFS trains have distributed power.
