What is a DMU?

[Caravanman]

Hi,
I seek definition of dmu... My understanding of the term is that the coaches that made up the dmu were not meant to be seperated, that the engine, cabs, passenger facilities were distributed throughout the "unit" and that it operated as a self contained train, either alone or coupled to more dmu's.
An alternative description which says that the dmu refers to the engine component only is suggested... a dmu set with only only one engine is not a dmu?

So, does the "multiple unit" refer to the integrated system, or just to the multiple use of engines?

Edwin

 

[Bigfoot]

Simplest answer is to think of it as an EMU with diesel engines instead of shoes or pantograph. They are both MU purely power supply that is different.

 

[jopsuk]

The exact definition is wooly. It is not well defined. Do not expect a hard and fast definition. There is not one. For MU and DMU differential speed restrictions Network Rail lists what is meant.

The classic that causes massive enthusiast arguments is that at introduction- and still in some Network Rail documents- the Intercity 125 High Speed Train was classified as a DMU by British Rail as Class 253 & 254.

 

[WillPS]

2xx is DEMU, I thought?

 

[swt_passenger]

2xx is DEMU, I thought?

DEMU (and DHMU (Hydraulic) and DMMU (Mechanical)) are usually just considered subsets of the DMU type, denoting the transmission detail. Besides, the question here is more about the meaning of 'multiple'...

Ellis's encyclopedia reckons 'multiple' refers to the provision of cabs at both ends of either a single vehicle or a set of vehicles.

 

[jopsuk]

To a certain extent it's simpler to define what a locos and coaches are- and accept that everything else is a so-called multiple unit. It's why the edge case of the HST causes so much pointless upset- originally the power cars were considered 43xxx series carriages, not "Class 43 locomotives"

 

[edwin_m]

The definition suggests the ability to work in multiple although that could possibly be just having several engines within a (nominally) fixed formation set like an HST. The GWR and other vehicles that couldn't run in multiple seem to have been described as Railcars.

 

[61653 HTAFC]

Literally:
Diesel
Multiple
Unit

So arguably, a 153/121/122 is a diesel railcar, rather than a DMU, though they are thankfully able to work in multiple with other units...

 

[jopsuk]

"Multiple unit" seems to cover various permutations:
With modern DMUs on the british network, it so happens that all of them have every car powered and they're all able to (and indeed, do) operate in multiple with at least units of the same class.
With current EMUs, there's a bit more variety. The "Mark 3 based" units all have a single powered coach. The older "PEP-derived" and most newer classes have several powered coaches- though I'm fairly sure all current mainline units have at least one trailer. However, whilst they can couple to rescue, Class 390 (and forthcoming class 700) can't operate in multiple.

The old southern region Mark 1 EMUs (and to some extent, the DEMUs too) were quite flexible and could be reformed, even using Mark 1 carriages with a little bit of work. They even had MUs without any motors/engines whatsoever- i.e. sets of 3/4 carriages with a driving cab at either end and control wiring, compatible with both EMUs and locos. Even "normal" stock was often formed into semi-permanent rakes, either full length trains or sub-units.

Then these days the "loco-hauled" trains on the GEML, ECML and Chiltern operate as fixed formations with remarshalling avoided if possible.

and then you've got HSTs. Which are both and neither top-and-tailed loco + coaches and DMUs. And were partly a development from the Edinburgh-Glasgow Class 27 top & tail, and partly from the Blue Pullman DMUs

Pigeon holes! everyone wants pigeon holes. Reality is more complex.

 

[455driver]

The Southern did a bit more than that, it was possible to use a 73 as a brake translator so you could have the front of the train using air brakes, then the 73, the rest of the train using vacuum brakes.

Okay it wasn't done very often but the (proper) standardisation meant that nearly everything could multi up, unlike the eff up we have got now such is the 'modern' way!

I have also seen the 4TC sets at Exeter with a 33/1 providing the power from the back.

 

[GM228]

Literally:
Diesel
Multiple
Unit

So arguably, a 153/121/122 is a diesel railcar, rather than a DMU, though they are thankfully able to work in multiple with other units...

I think they were railbus's/people carriers etc as opposed to railcars/DMUs if they couldn't couple or operate with other units, add couplers to the 139s and they become a DMU!

Railcar and DMU generally are the same, railcar is just the term for a self-propelled vehicle AFAIK, a 153 for example is both a railcar and a DMU, a 139 is a railcar, but not a DMU!

 

[jopsuk]

"Railcar" is used in eg the US or Australia for units with more than one vehicle. Just to muddy the waters further.

 

[GM228]

"Railcar" is used in eg the US or Australia for units with more than one vehicle. Just to muddy the waters further.

No, in the US a railcar is a term for any item of rolling stock on the rails usually just shortened to car! In Australia it is the same meaning as the UK!

 

[SpacePhoenix]

Perhaps one definition could be a train that can couple to another with the two then function as one train. But that would probably then mean that HSTs and 225s weren't multiple units

 

[D365]

Perhaps one definition could be a train that can couple to another with the two then function as one train. But that would probably then mean that HSTs and 225s weren't multiple units

The IC225 was never considered a 'multiple unit' train. EMUs like the Class 378, Class 700, Class 345 and the Moorgate replacement stock don't/won't couple in service either. jopsuk I believe has the best explanation.

 

[southern442]

No, in the US a railcar is a term for any item of rolling stock on the rails usually just shortened to car! In Australia it is the same meaning as the UK!

Am I correct in thinking that in Australia (or possibly another English-speaking country) the term 'railcar' is actually used to define any diesel multiple unit?

 

[edwin_m]

Am I correct in thinking that in Australia (or possibly another English-speaking country) the term 'railcar' is actually used to define any diesel multiple unit?

The term is widely used for the various DMU fleets in Ireland. Irish Rail website refers to the 22000 as an Intercity Railcar.

 

[Michael.Y]

Perhaps one definition could be a train that can couple to another with the two then function as one train. But that would probably then mean that HSTs and 225s weren't multiple units

Dead HSTs have been hauled by Class 43 power cars before now (connected nose to nose) so I guess that's working in multiple... all that's missing is 8 passenger carriages on the forward pair of 43s.

 

[edwin_m]

If the HST is dead then it isn't working in multiple with the extra power car(s) hauling it. I don't think the nose end of a HST has the necessary jumper cables for working in multiple from that end.