Push Pull

[Lewis]

The "Enterprise" (Irish cross border service) and several trains on the Rhaetian Railway have a loco at one end and a driving car at the other, resulting in a push pull train.
When the loco is pushing why does it not de-rail?
(trying this on a model railway results in a disaster)

 

[Nym]

If you mean model railway as in something around O gauge, it's because that stock doesn't have buffers, when pushing heavy rail you're pushing the buffers, I'm guessing on models you're pushing with the hooks.

 

[Peter Mugridge]

The scale size / weight and power / weight ratios are all wrong on a model railway, plus on a model the scale curve radius is far tighter than on the real thing.

So on the model you have something which is grossly overpowered, seriously underweight and accelerates like Concorde into over-tight curves... Quite apart from the inferior coupling gear that keeps locking up and over-riding.

 

[starrymarkb]

A lot of 1:1 scale Push-Pull stock uses rigid couplings as well. IIRC the most recent builds (like the OBB Railjet) have permanent bar couplings between the coaches

 

[O L Leigh]

It's down to weight and speed. Model railway coaches are very light to enable a loco to haul long trains, but the real things are much heavier. Also you have to be a bit more delicate on the power. The model couplings have nothing to do with it as they provide a function where trains can be pushed rather than pulled. Even propelling using the humble tension lock coupler permits this as you are pushing against the bars which act as buffers on the real thing.

If you really want to do this and make it look good, add some additional weight and don't be so generous with the power.

O L Leigh

 

[Crossover]

I have managed to run a train backwards with the engine at the back and 3 (or 4, I can't remember) wagons in front on Hornby. Generally it worked but when it met anything (strategically, I'll add) on the track and it didn't fair too well - nor did it fair well when it came to reverse set points and that also derailed it. Running engine first, hitting something it tended to push it out of the way moreso and getting to reverse set points, it just set them normal.

This was with a short train though but yes, generally the weight I would say is the main problem. Look at when James May did the Hornby thing and one of the teams added (unconnected) batteries to give the trailers some weight to stop ithem falling off the track

 

[Hydro]

A lot of 1:1 scale Push-Pull stock uses rigid couplings as well. IIRC the most recent builds (like the OBB Railjet) have permanent bar couplings between the coaches

The GatEx Mk.2's, when formed as 2/3 coach class 488 trailer sets, had permanent bar coupling between intermediate coaches.

 

[jopsuk]

The "Enterprise" (Irish cross border service) and several trains on the Rhaetian Railway have a loco at one end and a driving car at the other, resulting in a push pull train.
When the loco is pushing why does it not de-rail?
(trying this on a model railway results in a disaster)

In 1984 a Mark 2 DBSO leading a train of five Mark 3s propelled by a class 47 hit a cow at 85mph at Polmont, and derailed, killing 13 people. The aftermath of this led to the DVTs for Mark 3 stock on the west coast (now mainly on the Great Eastern, but also on the Chiltern mainline) and the Mark 4 stock on the East Coast being non-passenger carrying, and I think they're ballasted too?

However, there hadn't really been a problem on the Southern network, where various push-pull formations had run for many years- the Weymouth service for instance used "4TC" unpowered "multiple units". One or two of these were pushed from Waterloo by a powerful 4REP unit to Bournemouth, from where they were hauled to Weymouth by a class 33 and then pushed back to Bournemouth.

More generally, most older EMUs (including designs built into the late eighties such as 317-322, still running) used a single powered car in the middle with two or three trailers- so are always running with one or two trailer cars being pushed.

 

[TDK]

If you mean model railway as in something around O gauge, it's because that stock doesn't have buffers, when pushing heavy rail you're pushing the buffers, I'm guessing on models you're pushing with the hooks.

67's have auto couplers that can be used and the buffers are retracted when these are used, the coaches on the 67/Mk3/DVT combo including the DVT to coach are all buckeye couplings with buffers retracted - however if a loco is attched to coaches with a screw coupling you certainly need the buffers extended.

 

[sprinterguy]

While I know I have come to this discussion rather late and pretty much every facet of the subject has already been ably answered by other posters, I would just like to say that, even with the primitive tension lock couplers that 00 gauge model railways employ, there's no good reason why model trains should derail when being propelled on reasonably well laid, even track: In the past, five and six carriage class 90 + mark 3 + DVT, IC225 and HST formations (Given that the Hornby and Lima HSTs only have one motorised power car) have quite happily run on my layout for long periods of time without mishap over second radius, and occasionally at slower speed over first radius curves with the loco pushing at the rear. And I've seen full length IC225 and HST formations running without complaint with the loco at the rear on various exhibition layouts.

And on my current steam era layout, I regularly perform shunting manouvres that see up to about ten 4-wheel wagons being propelled over some complex pointwork and around curves of occasionally dubious radius and curvature (The far end of some of my sidings feature some bits of track that are, ahem, less than perfect, having been laid as a temporary measure when I had more pressing matters at hand but have never replaced so far!) with few problems. And that's sometimes with Hornby 0-4-0 locos, which are hardly renowned for their good quality slow speed control, but with a steady hand can provide some acceptable slow speed running.

As O L Leigh says, it's probably best to go easy on the speed!
--- old post above --- --- new post below ---

The scale size / weight and power / weight ratios are all wrong on a model railway, plus on a model the scale curve radius is far tighter than on the real thing.

It is certainly interesting to consider that if the weight of the real thing was scaled down in proportion, then a Hornby HST powercar would way round about a tonne! hock:

 

[LexyBoy]

It is certainly interesting to consider that if the weight of the real thing was scaled down in proportion, then a Hornby HST powercar would way round about a tonne! hock:

Actually it would be around 160 g, as mass scales with volume not length (7*10^7 g * 1/76^3 = 159.5 g). I'd guess that the unpowered model vehicles are rather lighter than their scale weight however.

125 mph would scale to 0.73 m/s, which is about half an average walking pace. Also bear in mind that any obstructions or defects in the track are going to be much more significant in the model - a 1 mm change in rail level where track joins would be 7.6 cm at full size - which would be rather alarming to see from trackside, let alone travel over!

 

[ChrisCooper]

Actually it would be around 160 g, as mass scales with volume not length (7*10^7 g * 1/76^3 = 159.5 g). I'd guess that the unpowered model vehicles are rather lighter than their scale weight however.

I was just thinking myself that whilst model locos are probably closer to scale weight, coaches are probably lighter, which will have an effect. In real life locos typically weight 3 or 4 times coaches (although with electric locos mostly used for push-pull it's closer to 2x), but I get the feeling that the difference is higher for models, afterall in both cases they want the locos as heavy as possible for traction and the coaches as light as possible, but it's much easier to save weight on model coaches than real life.

 

[hairyhandedfool]

In 1984 a Mark 2 DBSO leading a train of five Mark 3s propelled by a class 47 hit a cow at 85mph at Polmont, and derailed, killing 13 people. The aftermath of this led to the DVTs for Mark 3 stock on the west coast (now mainly on the Great Eastern, but also on the Chiltern mainline) and the Mark 4 stock on the East Coast being non-passenger carrying, and I think they're ballasted too?

However, there hadn't really been a problem on the Southern network, where various push-pull formations had run for many years- the Weymouth service for instance used "4TC" unpowered "multiple units". One or two of these were pushed from Waterloo by a powerful 4REP unit to Bournemouth, from where they were hauled to Weymouth by a class 33 and then pushed back to Bournemouth.

More generally, most older EMUs (including designs built into the late eighties such as 317-322, still running) used a single powered car in the middle with two or three trailers- so are always running with one or two trailer cars being pushed.

AFAIK, the restriction currently is that propelled leading vehicles can't carry passengers in the first third of the vehicle if they go over 100mph, so this restriction only applies to Voyagers/Meridians, Pendolinos and the Mk4 and Mk3 DVTs atm.

In terms of models, I found the Lima Mk3s to be far better for push-pull than any of the Hornby guff....I mean coaches....They are far too light.

This video was taken on one of my old layouts and is a specially fitted Hornby 86 (twin motor and extra weight), eight Lima Mk3s and a Hornby (Mk4) DVT.

 

[jopsuk]

AFAIK, the restriction currently is that propelled leading vehicles can't carry passengers in the first third of the vehicle if they go over 100mph, so this restriction only applies to Voyagers/Meridians, Pendolinos and the Mk4 and Mk3 DVTs atm.

It seems that restriction may go if the 350s are allowed to run in service at 110mph.

 

[hairyhandedfool]

I believe the 350s have DMSOs and are therefore not propelled.

 

[90019]

I believe the 350s have DMSOs and are therefore not propelled.

But so do Voyagers, Meridians and Pendolinos, so why would it apply to them and not to 350s?

 

[DaveNewcastle]

. . . . Polmont . . . .
The aftermath of this led to the DVTs for Mark 3 stock on the west coast (now mainly on the Great Eastern, but also on the Chiltern mainline) and the Mark 4 stock on the East Coast being non-passenger carrying, and I think they're ballasted too?

Yes.
My recollection of this is unreliable, but I seem to remember 8 tons of concrete under the floor of the DVTs used in '225 sets'.

Notwithstanding some of the comments in this thread about the derailment of models, when the EastCoast DVTs are being pushed through some of the rougher junctions and crossovers on the ECML at 125mph, then one does wonder about their stability! One of the worst is Temple Hirst Jn (which we've discussed recently on here), though this seems to be subject to a very welcome speed restriction between Temple Hirst and Balne these days.

 

[hairyhandedfool]

But so do Voyagers, Meridians and Pendolinos, so why would it apply to them and not to 350s?

That's a fair point, stupid me.

 

[IanXC]

AFAIK, the restriction currently is that propelled leading vehicles can't carry passengers in the first third of the vehicle if they go over 100mph, so this restriction only applies to Voyagers/Meridians, Pendolinos and the Mk4 and Mk3 DVTs atm.

180s and 395s presumably too. Looking at a 395, and the position of its first passenger door it doesn't seem to be as much as a third but I guess its in the sharply raked nose.

 

[jopsuk]

The driving cars of the 395s don't have motors- the only cars of the formation that don't!

So far though they only run at over 100mph on a far more secure railway with a very much lower chance of accidental or deliberate obstructions on the line

 

[IanXC]

The driving cars of the 395s don't have motors- the only cars of the formation that don't!

So far though they only run at over 100mph on a far more secure railway with a very much lower chance of accidental or deliberate obstructions on the line

Thanks, I hadn't thought about the nature of HS1 running or that 100mph would be the maximum on the classic lines.

 

[Tomnick]

I'd understood that the restriction on carrying passengers in the first third (or quarter?) of the leading vehicle applied to any train with a maximum speed above 100mph - nothing to do with whether it's being propelled. That would make sense, as the leading portion of the leading vehicle will form a 'crumple zone' for the protection of the occupants of the rest of the train.

If the concern was related to lighter vehicles becoming derailed or generally thrown about owing to the momentum of heavier vehicles behind, then surely passengers wouldn't be allowed anywhere in the unpowered vehicles at the leading end? Evidently there was such a concern following Polmont, hence the additional weight added.

 

[jopsuk]

Thanks, I hadn't thought about the nature of HS1 running or that 100mph would be the maximum on the classic lines.

They've never run on AC on "classic" lines- 100mph is the limit for all DC 3rd rail (to do with power supply, contact and "ramp" design at breaks in the 3rd rail). If the 350s are allowed to run at 110mph with no restrictions, then I would have thought that the 395s (or related design) would be cheerfully allowed to run at 125mph on the ECML (for example) subject to pantograph pressure and current draw restrictions (or lack thereof)