Isn't it more practical to have DMUs as opposed to pulling locomotives?

Question

This would be in regards to commuter passenger rail.
New Jersey Transit uses the pulling locomotive.
Long Island uses the DMU.
There must be some reason for the difference.

Answer 1

Good question - DMUs are excellent and very easy to join together in varying length formations, as well as that each coach has its own engine and/or traction motors which makes for a more reliable train (if one engine or motor fails the other(s) can still move the train along the track).
BUT...
DMUs are very expensive to buy - a train formed of locomotive and coaches can be shortened and coaches put in a siding, or lengthened using coaches from a siding or shed. These coaches, having no expensive engine and usually no expensive computer system and cabs, are much more economical to maintain for occasional use than keeping one or several multi-million dollar DMUs sat around for occasional heavy loading days.
However, DMUs usually cost less per seat per mile to run.

So presumably New Jersey Transit has worked out its business plan based on 100% use of expensive locomotives together with varying use of coaches, whereas Long Island's plan is based on varying use of DMUs, but where most are in traffic most or all of the time.
Incidentally another common problem with DMUs is a lack of through corridor connection between 'sets' - meaning lost revenue for the rail company unless a conductor is provided on each set.

Answer 2

I think it's best to have a running diesel on one end and a cab-control unit on the other. What's the point of having an engine on every unit? They're dirtier, they're noisier, and the power is spread out over the whole train. That might seem like a good thing, but have you ever seen pictures of Budd RDC units lashed together? Sometimes you get some pretty nasty slack action in those things!

The only safety concern that really strikes me concerning cab-control is the prospect of collisions with cab-control units leading. If a collision occurs with the locomotive shoving from the rear, the coaches are squeezed between the force of impact at the head and the driving force at the rear. Depending on what you hit and how fast you're going, that can make derailment more likely. Under normal circumstances with the locomotive leading, the force of the train is concentrated at the head end, and the coaches are just along for the ride. If you hit something, the power is right there at impact, and usually everyone on the train is okay.

Answer 3

Yes, it is. That is the reason why nearly all passenger trains in the UK from metro commuter trains to the fastest expresses are now either diesel or electric multiple units. If there is a locomotive, then the train can be driven from the rear by a driving trailer. This means trains can be driven into terminal stations and straight out again (after loading, of course) without any need for movements to place a locomotive on the leading end and to release locomotives that have brought trains into the terminal. For a while, because of safety fears, high speed trains were not allowed to carry passengers in the front and rear driving cars, so the 25 year old High Speed Trains have a notional 'loco' at each end (see http://web.ukonline.co.uk/cj.tolley/images/rail/98c21550.jpg), but the latest Pendolinos do not.(see http://www.railgallery.co.uk/mis/Pendolino_Ansty.jpg)

Answer 4

i thought long island railroad used electrical mu as well as the dual diesel/electric DM30AC

in any case, you are right. that's why the lirr has mostly 3rd rail. in some parts they need to switch to diesel. on the routes that are all electric, they run the multiple units. on the routes that are partly electric, they run the dual engines (pulling kind).