DMU v Loco Hauled

[Driverme10]

Yeah downhill most the way, you do drop down to about 20 as you go over top of hill near West Hampstead but usually braking from about 40 as you near Marylebone

 

[Billy A]

Why can't a 172 coast?

They're not designed to, is the short answer. DMUs usually use Voith hydrodynamic transmissions which transmit power hydraulically and are designed to freewheel when power is cut. The 172s use conventional ZF automatic gearboxes (they're the same type that were used on city buses) which don't do this. They're more efficient under acceleration because they lock the torque converter and transmit power mechanically for much of the time, but the lack of coasting rather negates this.
There's no technical reason why you couldn't incorporate a freewheel though.

 

[Bornin1980s]

What happens if a 172 suffers single engine failure?

 

[alexl92]

How much space does a loco take up?

In effect, one coach length. In other words, you can either have a 5-car DMU or 4 coaches + 1 loco in the same space.

On some services, losing one coahc worth of passenger space for a loco would cause havoc.

That said I prefer loco haulage/HST for the ride quality.

 

[squizzler]

Some things would be a lot easier if we had around six hundred 125mph capable unpowered coaches instead of all of the 180/ 220/ 221/ 222s - it'd be much easier to alter formation lengths, it'd be much easier to swap coaches between TOCs (compared to the saga of the "spare" Voyager power cars at Virgin). All of the small fleets of DMU can become awkward - see also 175s etc. But, since we are where we are, any fleet of unpowered coaches (like the TPE ones) is going to be another micro fleet which doesn't necessarily solve things.

For a good counterfactual look at Israel with all its uniform fleet of Twindexx double deck coaches and modern Stadler (nee Vossloh) locomotives. I presume the reason this works out for them has something to do with the network being brand new and designed as a vertically integrated system with the benefit of being able to optimise it for proven technology. A great contrast with the UK's legacy Victorian network which seems to encourage operators to scratch about with short formation DMU's.

 

[ac6000cw]

Going back a few decades, based on whole-life costs (purchase+maintenance+running costs), BR reckoned that above about 5-6 carriages concentrating all the power equipment in one vehicle (loco/power car) was cheaper than a DMU.

However the ability to better match train lengths to demand by making longer and shorter trains out of 2/3/4 car MUs during the day and over portions of long routes is one reason that (overall) the DMU solution may be cheaper to operate.

 

[Bletchleyite]

You can of course go hybrid - MUs with separate power cars, like Stadler FLIRTs or just LHCS with autocouplers. SBB basically treats LHCS as MUs, with fixed formation sets and fixed formation add-on short sets which are just plonked on the end at busy times often with the loco in the middle.

 

[HOOVER29]

I regularly use class 222 Meridians to travel from London to the East Midlands and I have no issues with them whatsoever. I barely notice the engine and they are very quick off the mark.

With loco haulage if the loco fails you are knackered with up to several hours delay. Lose an engine, maybe even two and the Meridian will still get you home.

I've always noticed/admired the way a class 222 gets underway from a stop. Leaving Leicester heading north they're away like a rat up a drainpipe.
Class 220/221's on the other hand seem very slow by comparison.
Are the 220/221's still downrated on the power?
If so then this might be the reason.
Also the 222's seem better put together & ride better compared to XC's main toys.

 

[ac6000cw]

You can of course go hybrid - MUs with separate power cars, like Stadler FLIRTs or just LHCS with autocouplers. SBB basically treats LHCS as MUs, with fixed formation sets and fixed formation add-on short sets which are just plonked on the end at busy times often with the loco in the middle.

Yes, but the Swiss (and others) still tend to have 'station pilot' locos (and staff) around at the larger stations, and don't have the same hangups we seem to have about propelling passenger trains for shunting purposes etc.

The days of splitting/joining Birmingham - Scotland loco-hauled trains at Carstairs, or propelling loaded loco-hauled trains into the 'other side' platforms at Inverness are long gone...(and I don't see them returning).

 

[43183]

I'm not sure about rail, but with road it's estimated that damage due to impact forces increases with the fourth power of pressure (weight divided by tyre contact area) - to the extent that a car does approximately 10,000 times the damage to road surface that a bicycle does. If the same holds true for rail, even roughly, then that 3 extra tonnes per axle of a 68 (21 tonnes per axle) versus a 185 (18 tonnes per axle) (which is an extreme example of a DMU as you well know) makes a heck of a difference. With something such as a class 220 at 11.5 tonnes (a 222 is similar) and the likes of Turbostars even lighter the differences becomes stark. The lighter coaches won't make up for the sheer pounding of the loco.

Going by the latest track access charges on the Network Rail website, a Class 185 costs 11.35 pence per vehicle mile, so 68.1 pence per mile for the 6 car. A class 68 isn't listed, but a class 67 is down as 58.47 pence per mile, and a mark 3 coach is 6.46 pence per mile (at least I think that's what that line means), so a total of 90.77 pence per mile for the loco and five coaches.

 

[Bletchleyite]

Yes, but the Swiss (and others) still tend to have 'station pilot' locos (and staff) around at the larger stations, and don't have the same hangups we seem to have about propelling passenger trains for shunting purposes etc.

The days of splitting/joining Birmingham - Scotland loco-hauled trains at Carstairs, or propelling loaded loco-hauled trains into the 'other side' platforms at Inverness are long gone...(and I don't see them returning).

Agreed, but you could have a "DMU" which is basically LHCS with autocouplers throughout that you operate just like a DMU, just that the locomotive is a separate vehicle.

The new Caledonian Sleeper stock will have autocouplers.

The 220s were originally going to be a one-ended Class 67 with 4 unpowered coaches (the same as the present powered ones).

 

[robbeech]

They're not designed to, is the short answer. DMUs usually use Voith hydrodynamic transmissions which transmit power hydraulically and are designed to freewheel when power is cut. The 172s use conventional ZF automatic gearboxes (they're the same type that were used on city buses) which don't do this. They're more efficient under acceleration because they lock the torque converter and transmit power mechanically for much of the time, but the lack of coasting rather negates this.
There's no technical reason why you couldn't incorporate a freewheel though.

Thankyou for answering my question at the same time, i assumed it would be this. Incorporating a freewheel means more serviceable parts, and more wear and tear on the solenoid(s) jumping in and out all the time, its negligible on a car but with a train i guess the level of torque is so high that it will cause excessive wear as the drive train locks to the engine output.

 

[Mordac]

Thankyou for answering my question at the same time, i assumed it would be this. Incorporating a freewheel means more serviceable parts, and more wear and tear on the solenoid(s) jumping in and out all the time, its negligible on a car but with a train i guess the level of torque is so high that it will cause excessive wear as the drive train locks to the engine output.

Can't you add a clutch?

 

[SpacePhoenix]

With loco hauled you either need to have a suitable DVT or the terminus stations need to have the facility for the loco to run round the train or have another loco take over. MUs don't have that restriction

 

[robbeech]

Can't you add a clutch?

Crikey, imagine the wear and tear on that.

 

[CosherB]

With loco hauled you either need to have a suitable DVT or the terminus stations need to have the facility for the loco to run round the train or have another loco take over. MUs don't have that restriction

Or top-and-tail, HST etc .......

 

[RichmondCommu]

Or top-and-tail, HST etc .......

I always think with top and tail you are basically wasting an asset.

 

[Bornin1980s]

It also means up to 100t unnecessary weight.

 

[SpacePhoenix]

Or top-and-tail, HST etc .......

Depending on signal positions, with top&tailing you might well have to deduct 2 coaches from the useable platform length

 

[ac6000cw]

Thankyou for answering my question at the same time, i assumed it would be this. Incorporating a freewheel means more serviceable parts, and more wear and tear on the solenoid(s) jumping in and out all the time, its negligible on a car but with a train i guess the level of torque is so high that it will cause excessive wear as the drive train locks to the engine output.

I would point out that most 1950's/60's era BR DMU powertrains consisted of an epicyclic semi-automatic gearbox with fluid coupling and freewheel, so it has been done before on UK DMU's.

 

[tbtc]

For a good counterfactual look at Israel with all its uniform fleet of Twindexx double deck coaches and modern Stadler (nee Vossloh) locomotives. I presume the reason this works out for them has something to do with the network being brand new and designed as a vertically integrated system with the benefit of being able to optimise it for proven technology. A great contrast with the UK's legacy Victorian network which seems to encourage operators to scratch about with short formation DMU's.

Interesting example.

I'd have no problem with loco-hauled if we were starting from scratch - I kind of envy countries who are able to start with a blank sheet of paper.

But with Victorian infrastructure that has been "modernised" (with "Middle Roads" and Headshunts and other luxuries squeezed out to make room for more platforms etc), I can't see any way of bringing back mainstream LHCS (other than a situation like TPE where a bespoke bid is a way of overcoming the potential stock shortages caused by not being able to get everything built in time by one manufacturer).

The days of splitting/joining Birmingham - Scotland loco-hauled trains at Carstairs, or propelling loaded loco-hauled trains into the 'other side' platforms at Inverness are long gone...(and I don't see them returning).

I could get misty eyed about my 1980s weekends on Carstairs station, back when you didn't have luxuries like RTT so had no idea of what the next service would be or what that light engine was up to - or even why that "Stratford" 47 was stabled in Lanarkshire - but I appreciate that those days aren't coming back

Services are busy enough in their own right (not to need complicated portion working - e.g. Scotland to Carlisle is often three trains per hour, so "Edinburgh" and "Glasgow" services are viable in their own right)...

...and more time critical (compared to Edinburgh - Carlisle passengers wasting fifteen minutes at Carstairs as their service came round the chord past the Penitentiary, through the station, headed to a siding at the north of the station, waited for the Glasgow portion to pass it then reversed back into the platform to couple up)...

...great fun to watch but operationally awkward and won't be coming back again.

With loco hauled you either need to have a suitable DVT or the terminus stations need to have the facility for the loco to run round the train or have another loco take over. MUs don't have that restriction

Or top-and-tail, HST etc .......

I always think with top and tail you are basically wasting an asset.

It also means up to 100t unnecessary weight.

Good points.

We don't have the luxury of taking up all of that platform space with two locomotives at our busier stations - there's a lot of waste.

Threads about bi-modes see people complaining that a bi-mode train will be carrying round unnecessary fuel tanks when running under the wires and will be carrying round unnecessary pantographs when running on diesel - yet people rarely complain about lugging a second locomotive hundreds of miles when we discuss HSTs/ topped and tailed LHCS etc.

 

[61653 HTAFC]

As other posters have said, BR had a longstanding policy relating to the cost crossover between the two modes as train length increased. In later years the other benefits of MU operation coupled with falling passenger numbers led to the railway as a whole favouring that operation.

The 68s though have been a step-change in performance by all accounts. In dry conditions they match or exceed 168s, and presumably they perform better with the old "leaves on the line" due to being heavier over the motored axles. Even so, the TransPennine sets were helped to come about due to a combination of circumstances:- the lack of capacity in the train building industry; the existence of the Mk5 production line due to the Caledonian Sleeper order; and the availability and proven performance of 68s. It probably also helped that the TP core is pretty much devoid of SP speeds so LHCS doesn't have the inherent disadvantage that it would on many other routes.

If any one of those conditions hadn't been present, the decision on the TransPennine fleet could well have been different.

 

[Marklund]

With loco hauled you either need to have a suitable DVT or the terminus stations need to have the facility for the loco to run round the train or have another loco take over. MUs don't have that restriction

Doesn't have to be a Driving Van Trailer either.
Other styles of driving vehicles are available too.

 

[mike57]

It probably also helped that the TP core is pretty much devoid of SP speeds so LHCS doesn't have the inherent disadvantage that it would on many other routes.

But 185's are not able to take advantage of higher speeds for 'Sprinters'. On Scarborough to York, where it looks like the loco hauled stock will end up the are long sections with higher speeds, up to 90mph for 'Sprinters' while 185's are limited to 75mph. If you look at the RPS times all the fastest times between York and Scarborough are for 158's.

They tried to kid us that the better acceleration of the 185's would make up for it, but the power to weight isn't as different 13.3hp per ton for 185 against 10.4hp per ton for the 158, bear in mind 185's have almost twice the power.

There are also differential speeds between Sheffield and Manchester as well as Leeds and Hull

 

[Bornin1980s]

Threads about bi-modes see people complaining that a bi-mode train will be carrying round unnecessary fuel tanks when running under the wires and will be carrying round unnecessary pantographs when running on diesel - yet people rarely complain about lugging a second locomotive hundreds of miles when we discuss HSTs/ topped and tailed LHCS etc.

The criticism does not apply to HSTs, as they require two power cars. I was thinking more of three coaches sandwiched between two Class 37s, one easily able to handle all the coaches, the other just adding 115 tons.

That said, I am actually a fan of bi-modes. If your station is electrified, you no longer get diesel fumes from every train to non electrified destinations. Such trains will use the wires wherever they are, rather than just smoking at them. If the wires fail, however, they will just carry on with diesel power.

Speaking of carrying on, no-one has answered my last question on 172s. Since their engines apparently can't be disengaged, would one be stranded if just one engine fails?

 

[Driverme10]

No but it would be out of service. Would be much slower than a other unit with an engine out I'd guess about third power

 

[robbeech]

I assume there is a dedicated neutral position, but whether under fault conditions this can be used on one engine only I do not know, but would also be interested in knowing.

 

[fowler9]

Purely from a selfish point of view and ignoring the efficiencies of multiple unit operation loco hauled was far more interesting purely from a rail enthusiast point of view in my opinion. Not saying that units aren't interesting, just that there obviously used to be far more going on.

 

[LNW-GW Joint]

So why the proliferation of 'long' DMU's, 7 car 222's, 4/5 car 220/1 5 car 180...

You'll be overjoyed to learn that the Hitachi bi-modes will be further proliferating the long DMU trend.
In the doubled-up 10-car form to be used in the West Country and elsewhere off the wires, they will have 6 powered vehicles, and be 10% longer than a 10-car Voyager.

Track access charges are the main driver towards DMU operation.
I don't think any other country tries to operate fast (125mph) loco-hauled diesel trains.
In any case, there aren't any 125mph locos. TPE will only run at 100mph with 68s.
Class 67 was notionally designed for 125mph but never ran regularly at that speed; I think it is limited to 110mph now, if that.

 

[jonhewes]

With loco hauled you either need to have a suitable DVT or the terminus stations need to have the facility for the loco to run round the train or have another loco take over. MUs don't have that restriction

I don't think that a modern loco-hauled trainset has this restriction either. Most European loco-hauled trainsets use driving trailers, rather than DVTs. A driving trailer is essentially a normal passenger carriage with a cab built into one end. An example can be seen here

The use of MKIII and MKIV DVTs is inefficient, as they do not carry passengers. At the time they were built, the Polmont rail crash was a recent memory, so I suspect that this may have played a factor in BR building DVTs, rather than DTs.