Could compatible EMUs and DMUs in theory run in multiple?

[nw1]

This thread would have been more relevant in the days before bi-modes, but I'm just interested. Could compatible EMUs and DMUs have run in multiple?

Imagine an alternate reality in which BR was not privatised and the Southern Region slam-door EMUs and DEMUs were replaced in the early 2000s by a compatible, similar-design fleet of Networkers, including both electric and diesel variants.

Imagine also that they decided to third-rail electrify to Salisbury. Under such a compatible fleet, could you have had a 4DMU + 4EMU work to Salisbury (with the DMU being propelled by the EMU, 4TC-like), then divide at Salisbury with the DMU going forward to Exeter under its own power?

Or, could you have had a 4EMU + 4DMU work to Oxted (the EMU pulling the DMU), splitting, with the EMU working to East Grinstead and the DMU to Uckfield?

This was never done but I'm wondering whether the logistics of the privatisation model had anything to do with this. So basically, in theory, if the right units had been in existence (EMUs and DMUs of similar design and compatible with each other), could these kind of operations have taken place?

 

[Towers]

It would surely depend entirely on the wiring, and whether or not the trains were designed to work together? The main issue would be the appropriate adjustment of performance so that the EMU was accelerating at a similar rate to the slower DMU. I'm told the Southern Region had worked out how to do this, and so you could have a diesel 33 attached to a train of MK1 EMUs and the whole lot powering together smoothly.

The Southern Region of course were particularly savvy in ensuring that their stock was compatible, something that the fragmented modern railway could learn an awful lot from.

 

[nw1]

It would surely depend entirely on the wiring, and whether or not the trains were designed to work together? The main issue would be the appropriate adjustment of performance so that the EMU was accelerating at a similar rate to the slower DMU. I'm told the Southern Region had worked out how to do this, and so you could have a 33 attached to EMUs and the whole lot powering together smoothly.

The Southern Region of course were particularly savvy in ensuring that their stock was compatible, something that the fragmented modern railway could learn an awful lot from.

The assumption would be that the trains are designed to work together. Wondering whether this (DMUs being pushed/pulled by EMUs over electrified sections) would have been a viable option which could have allowed more use of electric power over electric lines without electrifying everything?

I know that this happened with TCs (I was aware of the workings of REPs and TCs as long ago as 1983), but wondering if there was any fundamental reason why it could not be done with a DMU as opposed to a TC?

 

[Towers]

The assumption would be that the trains are designed to work together. Wondering whether this (DMUs being pushed/pulled by EMUs over electrified sections) would have been a viable option which could have allowed more use of electric power over electric lines without electrifying everything?

I know that this happened with TCs (I was aware of the workings of REPs and TCs as long ago as 1983), but wondering if there was any fundamental reason why it could not be done with a DMU as opposed to a TC?

The REPs of course were very powerful things, designed specifically to tow 8 coaches of unpowered TC about, and using pretty rugged, old school technology. I'm not a techy type, but I don't know what the load towing attributes of early 90s EMU traction kit would have been like? Obviously a DMU is a pretty heavy thing too, far moreso than a train of essentially MK1 coaches. The EMUs would need to have been pretty gutsy! There's also the side issue of traction current, it's well known that plenty of places across the Southern struggle a bit to meet the supply needs of modern era EMUs, so that might have raised its head as a problem in some places.

 

[spyinthesky]

Under such a compatible fleet, could you have had a 4DMU + 4EMU work to Salisbury (with the DMU being propelled by the EMU, 4TC-like), then divide at Salisbury with the DMU going forward to Exeter under its own power?

I imagine it would have had to split long before Salisbury, Basingstoke more like.

 

[swt_passenger]

I imagine it would have had to split long before Salisbury, Basingstoke more like.

Yes, but in this hypothetical scenario they had already got third rail extended to Salisbury, as set up in post #1

 

[spyinthesky]

Got it now

Yes, but in this hypothetical scenario they had already got third rail extended to Salisbury, as set up in post

 

[JonathanH]

There were plans for 210+317 to run in multiple but no production class 210s were built.

On the modern day railway 800+801 run in multiple with 800s towing / propelling 801s on non-electrified diversions.

The problem seems to be that no suitable fleet has ever been created with the suitable compatability, not that it would be impossible to do it.

 

[bgstrowger]

When the SR refurbished that Class 205, it was rewired in such a way to be compatible with their EMUs.

I think the Class 210s were designed to be compatible with an EMU too for beyond the wires running.

 

[swt_passenger]

I think the idea of a matched pair of pure EMU and a bi-mode DEMU is the best solution to the “Salisbury type” route, just in the same way a bimode IET can run in multiple with an electric one. But in the normal timetable not every split at Salisbury is 50/50, eg a pair of five cars might not suit the relative capacities needed either side of the split point.

 

[mmh]

It would surely depend entirely on the wiring, and whether or not the trains were designed to work together? The main issue would be the appropriate adjustment of performance so that the EMU was accelerating at a similar rate to the slower DMU. I'm told the Southern Region had worked out how to do this, and so you could have a diesel 33 attached to a train of MK1 EMUs and the whole lot powering together smoothly.

Presumably this is a fairly trivial issue, else you wouldn't have so many differently powered DMUs able to be work in multiple with each other since the year dot.

Feels like the bigger issue is being able to haul the unpowered sets with the power of the powered sets. An EMU/DMU combo would always have at least one "dead" unit - if you're going to keep the DMU running while it's on an electrified line, what's the point? Unless to split at some point and have the DMU serve an unelectrified route on its own.

 

[etr221]

My understanding is that the late '60s SR mu system - used on REPs, TCs, VEPs, etc - also included 73s and the PP variant of class 33; and (IIRC) that at one time there were trains from Salisbury and Southampton that combined at Basingstoke and continued to Waterloo VEP+TC+33. I think the only reason that the SR demus weren't includable was that they had been built a bit earlier, and 'not that way'.

There's no reason why - as long as they're compatable - different mode traction shouldn't work together.

Some preserved American steam locos have been fitted with diesel control stands, to control assisting diesels behind them...

 

[D6975]

Feels like the bigger issue is being able to haul the unpowered sets with the power of the powered sets. An EMU/DMU combo would always have at least one "dead" unit - if you're going to keep the DMU running while it's on an electrified line, what's the point? Unless to split at some point and have the DMU serve an unelectrified route on its own.

Not if your intention is to run DMU/EMU combos right from the start. You design the DMU traction motors to run off the electric from the EMU when on OHLE sections of line.

 

[Ashley Hill]

Can a pair of IETs run under OHLE with one on electric and the other on diesel?

 

[XAM2175]

I've mentioned previously that it's done multiple times a day in Denmark - trains formed of a combination of IC3 DMUs and IR4 EMUs run in normal passenger service without any fuss. The IR4 units simply limit their performance to match that of the IC3s when they couple.

Of course, as others have noted, it relies on the purchaser specifying that they want this ability.

 

[mmh]

Not if your intention is to run DMU/EMU combos right from the start. You design the DMU traction motors to run off the electric from the EMU when on OHLE sections of line.

That would need some sort of 25kv bus through the coupling connections. I think there'd be lots of concerns over that!

 

[D6975]

That would need some sort of 25kv bus through the coupling connections. I think there'd be lots of concerns over that!

Why? The bus would only need to be at the voltage presented to the traction motors.

 

[londontransit]

My understanding is that the late '60s SR mu system - used on REPs, TCs, VEPs, etc - also included 73s and the PP variant of class 33; and (IIRC) that at one time there were trains from Salisbury and Southampton that combined at Basingstoke and continued to Waterloo VEP+TC+33. I think the only reason that the SR demus weren't includable was that they had been built a bit earlier, and 'not that way'.

There's no reason why - as long as they're compatable - different mode traction shouldn't work together.

Some preserved American steam locos have been fitted with diesel control stands, to control assisting diesels behind them...

Maybe off topic however the Milwaukee Road's electrics (the Little Joes for example) were wired to fully control the rakes of diesel locomotives (and their long freights) the electrics were hauling over its mountainous lines through Montana and Idaho. This saved having to detach the diesels on the electrified sections of the CMStP&P Railroad.

 

[nw1]

I've mentioned previously that it's done multiple times a day in Denmark - trains formed of a combination of IC3 DMUs and IR4 EMUs run in normal passenger service without any fuss. The IR4 units simply limit their performance to match that of the IC3s when they couple.

Of course, as others have noted, it relies on the purchaser specifying that they want this ability.

Interesting, so are the IC3s 'dead' when in multiple with the IR4s or are they running under diesel power? Even if the latter, there are still potential operational advantages: going back to the theoretical 'electrified Salisbury' case, you could have the DMU/EMU combo split at Salisbury and then the EMU could head on its own back to London if there was no imminent up Exeter to couple to.

If it's in theory possible, even with the heavier weight of a DMU compared to a TC, I do wonder whether this solution would have been adopted for NSE (Southern Region).

(I mentioned Networkers but I'm not insisting on the Networker design, could be potentially any design in existence by 2004, that being the date of replacement of the slam-door stock).

 

[Towers]

Can a pair of IETs run under OHLE with one on electric and the other on diesel?

I don't know, but I would very much doubt it. It may be possible via some sort of complex faffing, but I'm pretty sure it isn't avilable to the driver as an option. If you think about it, there isn't really any reason why it would be; there aren't any diesel-only IETs, so as long as one set in multiple has a pan up there would never be a need to run the other set on diesel.

== Doublepost prevention - post automatically merged: 29 Jan 2022 ==

Presumably this is a fairly trivial issue, else you wouldn't have so many differently powered DMUs able to be work in multiple with each other since the year dot.

The acceleration curve on DMUs is, I would think, broadly similar enough that varying classes can operate together in multiple without issue. However I would imagine if you combined a Networker and a Turbo, you would likely get yourself a very lumpy ride under acceleration as the EMU bounces off of its wheezing diesel counterpart departing every stop!

 

[nw1]

I think the idea of a matched pair of pure EMU and a bi-mode DEMU is the best solution to the “Salisbury type” route, just in the same way a bimode IET can run in multiple with an electric one. But in the normal timetable not every split at Salisbury is 50/50, eg a pair of five cars might not suit the relative capacities needed either side of the split point.

Yes, nowadays that would make sense. (The 'uneven split' could always be accounted for with different combinations depending on the service, e.g. 4Bimode + 4EMU as the standard, 4Bimode + 8EMU for an evening peak service with most passengers for Basingstoke/Andover/Salisbury, or 8Bimode + 4EMU for a Friday afternoon or summer service with heavy loadings down to Exeter).

 

[XAM2175]

Interesting, so are the IC3s 'dead' when in multiple with the IR4s or are they running under diesel power?

They run under their own power (noting that they need to run their engines for 'hotel' power regardless).

 

[AM9]

I can't see an reason why all multiple units couldn't have complete compatibility through their control interfaces. In posts by many (presumably) drivers here there has been mentionng of the normal driving control on trains, i.e. a four step controller. Many years ago not long after I joined RUK, there was a discussion about coupling DMUs where the power curve was quite different, - I think it might have been about class 142s and 170s which apparently are compatible. I asked whether the four steps on a controller corresponded to an absolute power level from the drive system or whether it was proportion of what the traction system was capable of. The answer was the latter, i.e. even when the power available was unequal, it would be shared betwen the two units. If the lowest power unit is at the front it would be given a slight push and if at the rear, it would be pulled a little more than its own system would provide. That's perfectly logical as there's no guarantee that all MUs will be in full working order when running in multiple. Sometimes an engine may have shut down so a pair of class 150s might have one 2-car unit with both engines running and one with only one. That's 50% mismatch, a far greater difference than between a typical DMU and a similar purpose EMU.
The ultimate proof is the 319/769 situation. A 319 has about 9.3hp per tonne, a 769 has about 8.4hp per tonne (EMU) and 6.6hp per tonne (Diesel) yet they can (and have I believe) run perfectly well together in both modes.

 

[nw1]

They run under their own power (noting that they need to run their engines for 'hotel' power regardless).

So by "own power" do you mean the IC3's diesel engine? Or the IR4 providing the power to the power system? Is there any saving in diesel consumption by being coupled to the IR4?

(Sorry if this is a silly question).

 

[XAM2175]

So by "own power" do you mean the IC3's diesel engine? Or the IR4 providing the power to the power system? Is there any saving in diesel consumption by being coupled to the IR4?

I mean that the IC3s have their engines running and delivering tractive effort to their mechanical transmissions. There's no traction power or heating/lighting supply passed through the coupler; only air and the multiple-working control signals. Perhaps this might have been done had they been designed at the same time, but the IC3s were designed and built before the IR4s.

The primary purpose of the multiple working isn't to save diesel - towing the IC3s dead would make the IR4s vastly slower, don't forget - but rather to allow portion working where the electric unit is dropped off at the end of the wires and the diesel unit carries on.

There is an additional bonus in that they can mix-and-match the two fleets for ad-hoc strengthening; for example, a service usually formed of a single IR4 can have a spare IC3 dropped onto it as and when required.

 

[43096]

Why? The bus would only need to be at the voltage presented to the traction motors.

What about the amps. Have you thought about how much power would be needed? Traction cables are pretty hefty bits of cable for a reason.

 

[D6975]

I was asked about the voltage...
The amps would indeed be considerable, but then I didn't suggest using 16/02.
The cables would be quite hefty, but it's not impossible to do.

 

[JohnElliott]

The original SR multiple units had a bus line between units at line voltage IIRC. So did the first few EPBs.

 

[edwin_m]

I was asked about the voltage...
The amps would indeed be considerable, but then I didn't suggest using 16/02.
The cables would be quite hefty, but it's not impossible to do.

As far as I'm aware there has never been a coupler (automatic or manual) that can be coupled and uncoupled in service but can also carry the voltage and current needed for traction purposes.

It's perfectly acceptable for a unit to pull or push another one completely dead or (for example) for a 91 to push its entire train at 140mph. Coupling two units with somewhat different performance would be less severe than this worst case.

 

[Irascible]

So by "own power" do you mean the IC3's diesel engine? Or the IR4 providing the power to the power system? Is there any saving in diesel consumption by being coupled to the IR4?

(Sorry if this is a silly question).

If you just used the diesels for acceleration and possibly hill climbing if there was something particuarily steep, shutting them off & cruising just on the electric might work. Treat the whole combination as a hybrid.