Brake failure query

[Telcontar]

I (finally) saw Spiderman 2 last night, and the film depicted a runaway Chicago 'L' train (but not in Chicago) with a clueless driver who didn't know what to do after the controls were smashed up (by the bad guy) and the train decided to accelerate out of control (as they do).

A UK driver explained on Yahoo! answers that if for any reason you can't de-energise the brake continuity wire you can cut all the circuit breakers including auxiliary and that will de-energise the wire and the brakes will apply. (Reported to be true, after someone miswired the brake control handle during a service!)

My question then: what does apply the brakes under these circumstances? Is there a fail-safe whereby the brakes are held off by electric power, allowing them to be automatically applied when all power is lost completely?

(If you expect Spiderman's arms to have the tensile strength to stop a couple of hundred tons of train heading towards a mid-air middle-of-nowhere buffer stop placed on the precipice, then you can't really argue against having a train driver who has no idea how to stop a runaway train that has all of its air!)

 

[yorkie]

There is so much wrong with that it's difficult to know where to start, but there is a current thread discussing brakes in the Caledonian Sleeper thread (from post #25 onwards) and also a Wikipedia article on air brakes.

Brakes will automatically apply if a driver does not cancel the regular vigilance warning device within a certain period of time, or if the train goes too fast through speed restrictions, or fails to slow down approaching adverse signals, or fails to acknowledge adverse signals. See TPWS & AWS.

The protections in place in the USA will, of course, be different to here but I doubt none of these protections exist, and therefore the entire scene can be disregarded as pure fiction.

 

[Telcontar]

I'm just referring to EPB in UK multiple unit stock and what would actually happen if you cut all power to the train including the battery – how does an unpowered train apply the brakes when they're electrically controlled instead of air controlled?

Yes, the film was absurd – the passengers didn't even activate the passcom!

 

[DXMachina]

well I guess the idea would be bias - the system being designed so by default the brakes are applied, Ie the opposite of how a car is designed.

if the brakes were designed with HUGE springs to keep the pads locked down on the disks or on the tread of the wheel, and then an electric motor pulls them off it, then cutting the power would cause a full brake application and the train can only move with the brake motor under power.

Although my understanding is this is whats done but its all pneumatic / vacuum pressure not electric
--- old post above --- --- new post below ---
come to think of it doesnt EPB stand for Electro-PNEUMATIC Brakes?

 

[Telcontar]

Yes: classic air brakes use train line pressure to signal state changes, while EPB maintains permanent main reservoir pressure and controls the brake state electrically through a group of wires. Classic air brakes detect failures through the triple valve; EPB detects loss of main reservoir pressure by air pressure–operated switches that interrupt the brake continuity loop when pressure drops.

It's Fast Reg's answer here that I'm curious about, in terms of the procedure followed to force brake application by cutting power. I'm just wondering what causes the brakes to apply without power.

 

[John55]

well I guess the idea would be bias - the system being designed so by default the brakes are applied, Ie the opposite of how a car is designed.

if the brakes were designed with HUGE springs to keep the pads locked down on the disks or on the tread of the wheel, and then an electric motor pulls them off it, then cutting the power would cause a full brake application and the train can only move with the brake motor under power.

Although my understanding is this is whats done but its all pneumatic / vacuum pressure not electric
--- old post above --- --- new post below ---
come to think of it doesnt EPB stand for Electro-PNEUMATIC Brakes?

Try this link for a description of the principles behind EP braking.

http://www.railway-technical.com/ep-brakes.shtml

 

[DXMachina]

I'm just wondering what causes the brakes to apply without power.

I think the point is that the power is required to de-apply the brakes and keep them off. Without power the brakes will always be on. Try to understand it that the brakes are always on and power takes them off.. talk of the brakes 'applying without power' is a misunderstanding

Applying without power is what the brakes are designed to do. Power is required for them not to be applied

 

[OuterDistant]

God, I drove the missus mad last night watching this!

"Look at that! It must be the only train in the world with no deadman's control"!
"So the trip stops aren't working, either?"
"Why is an elevated train doing about 120mph?"
etc.

 

[GB]

God, I drove the missus mad last night watching this!

"Look at that! It must be the only train in the world with no deadman's control"!
"So the trip stops aren't working, either?"
"Why is an elevated train doing about 120mph?"
etc.

I find these sorts of arguments quite amusing. The pendants picking on the technicalities which is quite clearly Hollywood artistic license and completely glossing over the premise of the story which in this case revolves around someone being bitten by a spider and receiving super powers.

 

[OuterDistant]

But in the Spider Man 2 universe you'd need dozens of people with super powers, given how dangerously their infrastructure is designed :P

 

[GB]

 

[Telcontar]

I think the point is that the power is required to de-apply the brakes and keep them off. Without power the brakes will always be on.

Not with ordinary air brakes – the compressor charges up a reservoir in each vehicle via the train line (end-to-end air hose) and that limits the braking capacity. If you apply the brakes too many times, you willl run out of air. Not likely with UK passenger trains, but with a long freight train you have a lot of brake reservoirs to refill and this is a real issue in the US. No dead man's handle will save you if that happens.

I need to read up more on EPB in case it does work in reverse; that was not my understanding, but I could easily be wrong.
--- old post above --- --- new post below ---

"Look at that! It must be the only train in the world with no deadman's control"!
"So the trip stops aren't working, either?"
"Why is an elevated train doing about 120mph?"
etc.

The train travels several miles past the last station to where the track just stops mid-air. Do elevated railways really have that much overrun? With the buffer stop right on the precipice? (I honestly don't know what's at the end of the line on elevated railways, but it seems weird.)

And yes, the original premise is wrong anyway: spiders don't extrude web, they pull it out. Spiders can't fire out strands of web through the air. Even real spiders struggle with climbing relatively smooth walls: I'm sure most people have seen a spider fall of a wall a some stage.

 

[GB]

Not likely with UK passenger trains, but with a long freight train you have a lot of brake reservoirs to refill and this is a real issue in the US. No dead man's handle will save you if that happens.

The brake distributor and auxiliary reservoir should prevent this. You should find that you won't be able to release the brake rather than the brake actually failing to apply.

 

[headshot119]

On Sprinters / Pacers the brake continuity is controlled via two train wires. Having no power in both wires causes a full brake application. So if for instance the train divides you'd get an automatic brake application on both portions.

 

[Wikipedia]

I'm just referring to EPB in UK multiple unit stock and what would actually happen if you cut all power to the train including the battery – how does an unpowered train apply the brakes when they're electrically controlled instead of air controlled?

Yes, the film was absurd – the passengers didn't even activate the passcom!

The EPB has an EP (Electro-Pneumatic) Brake. This is the system that is controlled electrically however this is not a fail safe system because a loss of power means no power to activate the valves to feed air to the cylinders. Therefore the fail safe "Westinghouse Automatic Air Brake" is superimposed over the EP Brake and provides the fail safe side of the system. As long as the train pipe has been charged up to approximately 70 psi before the power is removed from the train the driver or guard simply opens an emergency brake valve thus reducing the train pipe pressure to 0 psi causing a full emergency application.

http://www.railway-technical.com/ep-brakes.shtml This should explain all you need to know about the EP Side.

 

[iphone76]

I wish I could have read this last night whilst I was stuck for an hour and a half on the 18.32 from Liverpool Street to Wickford as the brakes applied themselves and locked.

Fortunately a fitter was eventually sent to sort us out.

 

[Telcontar]

The brake distributor and auxiliary reservoir should prevent this. You should find that you won't be able to release the brake rather than the brake actually failing to apply.

I presume you're referring to ECP or something else more sophisticated than the basic air brake, which requires full release in order to begin recharging.

The EPB has an EP (Electro-Pneumatic) Brake.

Sorry, I meant EPB as in the braking system, not the Southern designation.

http://www.railway-technical.com/ep-brakes.shtml This should explain all you need to know about the EP Side.

Love the MS Paint diagrams. The Wikipedia article implies that the train line is completely removed and replaced with the continuity wire, i.e. EPB is the sole braking system provided and that electric power replaces air pressure for detecting leaks (as in, loss of air pressure electrically selects emergency brake application by cutting the continuity wire). I don't know what applies the brakes if the whole train loses power, assuming anything does; perhaps there's just some valve gear that connects an emergency reservoir to the brake cylinder when power is lost.

Also, the Railway Technical article is based on systems with backup air brakes and infinite brake pressure control. If (Open)BVE is anything to go by, UK passenger trains have fixed braking pressures, e.g. 1, 2, Full for a typical EMU. The Railway Technical article is for a system where you can increase and decrease braking pressure at will instead of having fixed pressure notches, which goes back to the consideration of throttle notches vs continuous throttle.

 

[GB]

I presume you're referring to ECP or something else more sophisticated than the basic air brake, which requires full release in order to begin recharging.

Sorry don't know what ECP is but air brakes on UK freight wagons are fairly simple.

The brake cylinder and brake rigging is controlled by the brake distributor which is fed by the Aux Res and train pipe. The Aux Res is connected and fed by the train brake pipe and the train brake pipe is connected to the loco compressor which charges it to 5 Bar...this in turn charges and stabilizes the distributor for a full brake release.

As the driver applies the brake, air from one half of the distributor (which is split into two chambers by a diaphragm) is vented to atmosphere. The pressure change in the diaphragm allows air to travel from the Aux Res through to the brake cylinder there by applying the brakes. It only takes a small change in pressure to start applying the brakes.

When the driver recharges the brake pipe air is pumped back into the distributor and Aux Res equalizing and stabilizing it back to 5 bar. The recharging happens while the brakes are still applied (or in the process of releasing) as its the only way to get the brakes off in normal service.

If the driver keeps applying and releasing the brakes, while this is poor practice, there will still be plenty of air in the system for a full or emergency brake application as all the air is never fully used up during braking. The only issue is the more air you use, the longer it will take to recharge and release.

If a freight train is stabled and left for quite sometime then eventually all the air will leak out the system and there will be nothing other than hand brakes to hold the brakes on.

Vacuum brakes on the other hand are a totally different kettle of fish where you only have a finite supply of vacuum before you run out and the brakes won't apply.

 

[Jonfun]

Vacuum brakes on the other hand are a totally different kettle of fish where you only have a finite supply of vacuum before you run out and the brakes won't apply.

I might be misunderstanding you but surely you mean brakes won't release? If there has been vacuum in the train pipe and there is no longer, then the pressure on the train pipe side will be higher than that in the reservoirs on the individual vehicles thus resulting in a pressure differential keeping the brakes on (until such time as air leaks into the reservoir returning it to atmospheric pressure, although this is a slow process). Days where the vacuum brake system relied on vacuum being provided to *apply* the brakes are long gone!

 

[GB]

Possibly. I don't deal with vacuum brakes but have heard many stories from "old hands" of times where in "the good old days" vacuum brakes have failed to apply because the driver has used too much up and had to wait for it to rebuild to suitable levels.

 

[Telcontar]

If the driver keeps applying and releasing the brakes, while this is poor practice, there will still be plenty of air in the system for a full or emergency brake application as all the air is never fully used up during braking. The only issue is the more air you use, the longer it will take to recharge and release.

Apparently it depends whether you have a classic one-pipe braking system or a modern two-pipe braking system:

http://www.railway-technical.com/air-brakes.shtml#Two-Pipe-Systems

The two-pipe system seems to be like an air-driven version of EPB, using one pipe to supply air pressure and another pipe for communication. I'm not fully sure without studying it, but this apparently resolves the problem of running out of air Somehow.

 

[Wikipedia]

Also, the Railway Technical article is based on systems with backup air brakes and infinite brake pressure control. If (Open)BVE is anything to go by, UK passenger trains have fixed braking pressures, e.g. 1, 2, Full for a typical EMU. The Railway Technical article is for a system where you can increase and decrease braking pressure at will instead of having fixed pressure notches, which goes back to the consideration of throttle notches vs continuous throttle.

On more modern stock OpenBVE is correct, there are limited brake steps. However on the EP brake fitted to EPB's and similar stock (CEP, VEP, Thumper etc.) the brake is infinitely variable in theory, however the controllers aren't the most sensitive to minute movements of the handle. This video explains the EP fitted to EPB's and this is the newer style of EP brake.

 

[Jonfun]

Apparently it depends whether you have a classic one-pipe braking system or a modern two-pipe braking system:

http://www.railway-technical.com/air-brakes.shtml#Two-Pipe-Systems

The two-pipe system seems to be like an air-driven version of EPB, using one pipe to supply air pressure and another pipe for communication. I'm not fully sure without studying it, but this apparently resolves the problem of running out of air Somehow.

On a two pipe air brake system there's the train pipe and the main reservoir pipe which is constantly charged by the loco. When running with both pipes, the main reservoir pipe charges the auxiliary reservoirs in each vehicle thus keeping them topped up. A reduction in pressure along the train pipe will be detected by the distributor/triple valve which will admit a proportional amount of air from the auxiliary reservoirs to the brake cylinders, effecting a brake application. Thus if the train pipe is split the pressure drops and brakes are fully applied.

On a single pipe air brake system you only have the train pipe, and it is used to both charge the auxiliary reservoirs on each vehicle and to communicate brake applications down the train (all handled by the distributors/triple valves); with a charged reservoir it will work in the same way, however it does require time to charge between applications.

I'll be brutally honest, I've no idea what the type of EPB Wikipedia talks of is or does, but with the Westcode type of Electropneumatic Braking which this thread's about, you have a main reservoir pipe running along the train, which again is used to charge the aux reservoirs. Moving the brake handle to different notches varies what's going down the train wires, which I *believe* (but don't quite me as I don't have any direct involvement with this type of braking system, just what I've picked up) effects the stepped brake application by magnetic solenoidy type switches or similar in the brake equipment on each vehicle which admit different pressures of air into the brake cylinders. Loss of the train brake wire results in a full brake application as said magnetic switches fail safe (thus with nothing holding them closed they allow air pressure in to apply the brakes.

Basically, when you have a main reservoir pipe, it worries about keeping the reservoirs on each vehicle charged so the driver does not have to be concerned about making too many brake applications etc.

Returning to the vacuum brake situation, it is in theory possible to be left without brakes if the exhauster isn't running as you'd (very gradually) gain air into the reservoirs - of course I don't know from a driver's perspective whether the exhauster runs continuously in the release position (I would have imagined it does, but as long as the big red valve works okay, I don't need to know any better ).

 

[ChiefPlanner]

Possibly. I don't deal with vacuum brakes but have heard many stories from "old hands" of times where in "the good old days" vacuum brakes have failed to apply because the driver has used too much up and had to wait for it to rebuild to suitable levels.

Indeed possible , plus a brake handle left in the wrong position could cause a severe loss of vac , such that when you wanted to use the brake there was nothing there (this resulted in a head on between a DMU on a footex and 47111 at Preston in the late 70's , early 80's.

Also possible to have an intemediate blockage in the train vac pipe , giving a much reduced brake , again the cause of a severe accident at Ilford in the late 1940;s - the cause being a piece of newspaper sucked into the system.

2 pipe air brake , very much safer !

 

[Telcontar]

I'll be brutally honest, I've no idea what the type of EPB Wikipedia talks of is or does, but with the Westcode type of Electropneumatic Braking which this thread's about …

I just meant electropneumatic braking in general, not Southern's EPB stock. I don't know what the driver in the Yahoo! Answers question was driving, although having a solenoid-driven valve held in position by power appears to be the answer. The implication here is that if you switch off the train, the brakes come on.¹

I'm not quite I follow you here – is Westcode what all British stock uses, or just the Southern EPB stock and similar? I'm not sure whether you're saying that the Wikipedia article is complete rubbish (wouldn't be the first time or whether it relates to newer trains and you're only familiar with slammer-era designs. (The Wikipedia article doesn't tie in with the Railway Techical article on when the UK first got EPB – Wikipedia says BR Southern 1949, and Railway Technical said LU 1916. I guess it only reached heavy rail in 1949?)

I'll look at those YouTube videos tonight


¹ Reminds me: apparently the Aventra will switch itself on in the morning instead of the train being left on all night or some such – according to OpenBVE, with the 323 you just raise the pan, unlock the desk, accept AWS self test, release DRA and off you go, but clearly it's got a lot more complicated since the 90s. I've watched a video on starting a Sprinter (156?) and it's relatively short too (check stuff, start engines etc). No idea what's so wrong with Electrostars ...

 

[John55]

I just meant electropneumatic braking in general, not Southern's EPB stock. I don't know what the driver in the Yahoo! Answers question was driving, although having a solenoid-driven valve held in position by power appears to be the answer. The implication here is that if you switch off the train, the brakes come on.¹

I'm not quite I follow you here – is Westcode what all British stock uses, or just the Southern EPB stock and similar? I'm not sure whether you're saying that the Wikipedia article is complete rubbish (wouldn't be the first time or whether it relates to newer trains and you're only familiar with slammer-era designs. (The Wikipedia article doesn't tie in with the Railway Techical article on when the UK first got EPB – Wikipedia says BR Southern 1949, and Railway Technical said LU 1916. I guess it only reached heavy rail in 1949?)

The EP braked train was really a British Rail not a Southern Railway idea as the SR preferred to keep compatibility with earlier EMUs when it developed the 1936 type equipment (this was used in the 1940 4-SUBS etc). The Southern only had one experimental EP braked train in the 1930s. The 4DDs which were EP braked but not like the later 4-EPBs emerged in BR days.

The LMS and LNER had been using EP brakes on production trains, not experiments, since before world war 2 (1937/8 onwards) and I think LT used them from the mid - 1920s onwards.

Westcode braking is a relatively modern development in the UK and is presumably a proprietary system from Westinghouse B&SC although I believe other manufactures systems working on the same principle are called Westcode as well. I think it came in with the PEP derived trains in the 1970s. The key feature is a limited number of braking steps (3, 5 or 7) controlled by an electrical signal using a code of energised train wires rather than an infinitely variable set up as on the earlier equipments.

 

[Jonfun]

I just meant electropneumatic braking in general, not Southern's EPB stock. I don't know what the driver in the Yahoo! Answers question was driving, although having a solenoid-driven valve held in position by power appears to be the answer. The implication here is that if you switch off the train, the brakes come on.¹

I'm not quite I follow you here – is Westcode what all British stock uses, or just the Southern EPB stock and similar? I'm not sure whether you're saying that the Wikipedia article is complete rubbish (wouldn't be the first time or whether it relates to newer trains and you're only familiar with slammer-era designs. (The Wikipedia article doesn't tie in with the Railway Techical article on when the UK first got EPB – Wikipedia says BR Southern 1949, and Railway Technical said LU 1916. I guess it only reached heavy rail in 1949?)

Haha, we're at cross purposes here.

The westcode braking system isn't used by Southern Region slammers, that's a different type of system that I haven't the foggiest about. Westcode braking system is however found on stock such as pacers, sprinters, and that sort of thing - it is probably a similar system on more modern stock also. I won't say with 100% certainty that that's how it works as I don't have any direct involvement with it but that's the gist as it's been explained to me.

When I refer to Wikipedia, I'm referring to RailUK member "Wikipedia" who contributed to the thread above.

Hope it makes a bit more sense now!

 

[Telcontar]

This video explains the EP fitted to EPB's and this is the newer style of EP brake.

Wow. That second one is humbling – shows how little I really know

There I thought that you had to use only notches 1 and 2. From Anthony's BVE driver's guide for the Class 323 (well, it was back when I read it):

"As you pass beneath the next over bridge, you should begin to decelerate for the upcoming 20mph speed restriction at Alvechurch station. In the interests of passenger comfort, you should use brake handle positions B1 or B2. Only use B3 or EMG (Emergency Brake) if you have to!"

From some of the stiff braking on Class 319s, I figured that you had to brake gently on approach. (I also read that UK passenger trains don't have anti-lock brakes, but with wheelslip protection, they do …)

Instead, it's straight to brake notch 3 and let the train take the (computational) strain … (It's probably still true that you use steps 1 and 2 for speed control and speed reductions, but not for stopping at stations.)

When I refer to Wikipedia, I'm referring to RailUK member "Wikipedia" who contributed to the thread above.

Ohhh … I thoght you meant WP Electro-pneumatic brake system on British railway trains

The second video explains the electronics, but says nothing of how the physical braking system operates, and whether the classic air brake is retained for emergency. Certainly the brake handle no longer has the classic air brake notches, so maybe the only fail-safe now is the continuity wire and the solenoids.

 

[Wikipedia]

When I refer to Wikipedia, I'm referring to RailUK member "Wikipedia" who contributed to the thread above.

Haha ye it was probably a really bad choice of username for topics like this but it was a nickname from school that has just stuck!!