Locomotive braking system

[Derekstuart]

Hello

Could I ask any Driver/ Guard/ Engineer/ knowledgeable person a few questions that have been puzzling me, please?

1. Why have a separate locomotive brake control when, as I understand it, the train brake control will work fine when there's no train connected. I'm sure there's a good reason.

 

2. Why- at least on some classes- can you not reduce braking and instead have to release the brake and then re-apply to the appropriate level.

 

3. When coupling a locomotive to stock, I know you have to test the brakes. As I understand it this is applying the loco brake (see 1?) or hand brake, releasing the train brake then someone opening the train brake (red?) cock at the other end to see if air comes out and in doing so the Driver/ Guard notes the TB pressure dropping. Is that correct or is there anything else?

 

Many thanks in advance for this question of idle curiosity; I'm sure there's not only a good reason for 1 and 2.

 

Derek

[The Johnster]

The following is a very basic account, and there are all sorts of details left out.

 

Most locomotives, in fact TTBOMK all except GWR designs, do not have the automatic continuous train brake (vacuum or air, must be 'fail safe' and will apply in the event of a loss of vacuum or air pressure, the vac or air pump or exhauster hold the brakes off by pressure), but are stopped  by a steam brake (steam loco) or 'straight air' brake (diesel or electric loco).  In the early days, locomotives did not have brakes at all other than a handbrake, usually on the tender.  So, a diesel or electric loco will have an air compressor that will power an air brake to use when the loco is travelling light engine or working an unfitted train, as the handbrake is far too slow-acting and not powerful enough to be of much use out on the road at speed.  It's purpose is as a parking brake to secure the loco when it is berthed, shut down, and the straight air brake pressure has leaked away.  This is much the same system as is used on commercial road vehicles and buses.

 

Of course, the vacuum or air exhauster that releases the train brakes is aboard the loco, so the loco will have the necessary hoses and pipes, but the exhauster does not operate the brakes on the locomotive's wheels.  The train brake will work fine on the train when the loco is not connected, but it will not work on the locomotive's wheels.  I think that's question 1; there is a lot more to be said about this but I'm 71 years old and don't have the time...

 

As for releasing and re-applying the train brake,  this is because the brakes will take some time to release, as all systems involving flexible hoses and joints leak.  So, if the brake has been applied and you want to make a further application, you have to release the brake and start again if you want to simply slow the train down and not stop it.  A considerable amount of skill and experience is needed to do this without losing time, especially on trains with many vehicles, which of course have more joints, flexible hoses, valves, and pipe unions where leakage may be present.  In the early deisel days, even experienced drivers were sometimes caught out by this as the vacuum exhauster on a diesel locos was a lot less powerful than the steam-powered vacuum pumps they were used to, resulting in trains coming inintentionally to a stand in sections.  I can certainly remember working fully fitted vacuum trains with diesel locos in the 70s on which the air compresser was working flat out and continuously to keep the brakes off, and even quite hard sometime on air-braked trains.  When you apply the train brakes, they respond instantaneously,  but releasing them is a sometimes unpredictable delayed action response.  That's the very bare bones of question 2.

 

As for brake continuity tests, these are required whenever the continuity of a continuous automatic brake system is broken, air or vacuum, so it should be performed when a loco is coupled to the train and at any point at which the loco is changed and/or a vehicle is attached or detached.  There are difference methods of achieving this according to the exact circumstances.  On a fully fitted train without a brake van, the driver creates vacuum or air pressure in the system, releasing the automatic brakes, and you remove the hose (we called the bags on the WR) from it's stop on the trailing vehicle, or opened the red-painted air brake pipe cock on that vehicle.  You would hear an inrush of air into a vacuum system, or air escaping from an air system, and after the initial rush a lesser continuous noise as the loco's exhuaster tries to release the brakes, which it can't.  The man responsible for signing the train preparation certificate (me, the guard, back in the 70s) ensures that the brake blocks are firmly applied to the wheel rims on the last four axles of the train, by the highly technical means of kicking them* to make sure they do not move (when the are released, they hang pretty loose, cool dudes that they are).

 

I once performed a brake continuity test on a train of loaded MGR wagons that had been loaded with Polish coal for Didcot Power Station at Barry Docks.  Unloading from the ship had been a messy procedure with grab buckets on the normal cargo-handling dockside cranes, and a lot of the rather dusty crumbly coal was lying on the quayside.  The travelling shunter who'd accompanied us on to the dock system from Cadoxton coupled the loco on and connected the air hoses while I went to the back of the train to put the tail lamp on and do the brake test.  I'd heard the brakes releasing as I walked down the length of the train, put the lamp on, and opened the red train brake cock.

 

Nothing happened.  There should have been a rush of air coming out of the end of the pipe at 80psi, but nothing happened.  I examined the cock, and couldn't see anything wrong with it, then the end of the hose.  These have a spring loaded crosspiece valve that opens when they are connected to another hose, and this was invisible under a large amount of wet concrete-like coal slurry. 

 

What I did then may be regarded as a little foolish.  In an attempt to get rid of the slurry blocking the hose, I banged the end of it on the buffer beam.  There was a noise like a shotgun blast, and four quadrant-shaped plugs of solidified slurry went past my head close enough to ruffle my hair and tickle my ear, and disappeared skywards, looking as if they were heading for Orion and for all I know they still are, but the rate they were going they could have got there by now...  This incident taught me to treat blocked hoses with more respect in future, could have taken my head off not that that would have done much damage I suppose.  I got no sympathy whatsoever from the loco crew or the shunter, who though it was most amusing! 

 

Having nearly been decapitated, I was then able to complete the brake continuity test...

 

 

*you could also whack 'em with a brake stick if you had one handy.

 

.

 

Edited April 9, 2023 by The Johnster

[david.hill64]

A couple of points to add: In respect of question 2:

 

Trains that were fitted with a vacuum brake were able to have the brake application continually variable between off, full service and back to off. Early adaptations of the air brake relied on something called the triple valve to control the supply of air pressure to the brake cylinders from the reservoir on every vehicle. It is an essential characteristic of the triple valve that although you can demand increasing levels of brake application, once applied you must make a full release before reapplying. On more modern applications the triple valve has been replaced by a distributor. The distributor is fully proportional on both application and release. Moreover, on passenger stock it is linked to a load weighing valve in the suspension which then adjusts the brake cylinder air pressure according to both demand and vehicle weight. This keeps the brake force on every vehicle in a train the same to prevent longitudinal dynamic shocks.

 

In respect of question 1:

 

The vacuum brake is relatively slow to act. If you consider activities like coupling up to a train, having more precise control of the brakes is obviously desirable, even if early engines often lacked loco brakes. A direct steam brake is much quicker acting. On air braked locos, including steam locos, a direct air brake provides a relatively lightweight controllable option. Many years ago I had a cab ride on a Taiwan Railways loco hauled passenger train where triple valves were in use on the vehicles. On making a train brake application, the driver immediately dumped the brake on the loco and then as the train came to a halt applied the direct air brake. This caused the stock to gently buffer up to the loco, so that on starting the engine had the advantage of getting the first vehicles rolling before taking the whole train weight.

[Jeremy Cumberland]

To add a little to the previous replies.

 

1. Locomotives need a separate brake independent of the train brake so a train can be held on the loco brakes with the train brakes off. Train brakes take a long time to release, which is not very good when you are trying to pull away.

 

I think on all modern locomotives (from the BR modernisation scheme onwards), the loco brakes are also worked from the train brake valve, but it might not be as responsive when travelling light engine. On older locomotives, the loco brakes were often entirely separate and not worked by the train brake at all.

 

2. This (as far as I am aware) applies only to the Westinghouse automatic air brake, which used triple valves in each vehicle. The triple valve would progressively admit air from the carriage air reservoir to the brake cylinder as the pressure reduced in the train pipe, but to release the air from the brake cylinder, you needed to restore the train pipe to full pressure. As far as I am aware, British Rail never used triple valves on loco hauled stock, using distributors instead (which can be progressively released), but London Underground did, and I imagine the Great Eastern did too, and the Southern and its constituents for their electric trains. Electro-pneumatic braking  (which could be progressively released) was invented in the 1920s, and this relegated the Westinghouse brake to emergency use only on EMUs where it was fitted, and it remained in use till fail-safe electro-pneumatic brakes were invented in the 1970s. Apart from 1972 Mk 2 stock on the Bakerloo Line, I am not sure that anywhere in Britain still uses triple valves. The Isle of Wight Steam Reailway, perhaps.

 

3. Modern practice is to check that the brakes actually apply, as The Johnster says.

When I was first trained as a guard with vacuum brakes on a heritage railway, I was told to watch the vacuum gauge in the last vehicle, to see that the vacuum dropped when I worked the setter, but also to check that it rose again at a suitably brisk rate afterwards. It is watching the vacuum rise again that indicates there is no blockage in the train pipe - a common enough problem with vacuum systems with old, lined, bags (hoses) between vehicles. It was not unknown for the lining to become detached and to seal itself together.

[The Stationmaster]

The need for a separate brake on diesel and electric locos - apart from holding a train with teh auto brake fully released - is when running witha loose coupled train and only the loco and brakevan braking is available.

 

Ideally witha vacuum braked passenger train you should brake in such a way that when the train comes to a stand at an intermediate station stop, or when you run onto a speed restriction, the train brake vacuum is rising and the brake is either releasing or is fully released.  This, as mentioned above you will be at a stand at a station with no brakes applied on the train and you use the loco straight air brake to hold the train. While the vacuum brake is very quick to act when braking, mainly due to direct admission valves on each passenger vehicle which react to changes in the level of vacuum,  the brake is slow to release because air is only exhausted from the brake system, to recreate the vacuum, at the loco end.  hence the development ofthe driving technique of coming to a stand, or onto a restriction of speed on what is known asa rising brake (i.e. the level of vacuum is rising).

 

Driving like this required a good level of road knowledge, especially of braking points and knowing how the train is behaving braking wise,  to ensure that a train stopped in the right place or was reduced in speed by the right place with the brakes released.  It also gives passengers a smoother ride particularly when stopping.   You can also do it with an automatic air brake but I understand that it's not quite so easy with the older triple valve brake system.

 

So with an unfitted and, in particular vacuum braked, train the loco's straight air brake serves a very useful purpose.   and of course you need a brake to hold a loco when being stable befire applying the handbrake.

[bécasse]

In my youth, I regularly experienced Westinghouse-braked steam-hauled passenger services both on the Isle of Wight and out of Liverpool Street and my prevailing memory is how rapidly trains would come to a stand at the correct spot even when compared with the Southern Electric services that I habitually rode, and certainly a completely different experience to that on a vacuum-braked steam train. The stops, although rapid, were smooth and I never saw indications that they gave rise to any problems for passengers, some of whom, particularly on the IoW, would have been retrieving heavy items from overhead luggage racks at the time.

[JeremyC]

According to the 'BR Handbook for steam locomotive enginemen' some LNER locomotives (the A3 pacifics are specifically mentioned ) used vacuum brakes on the locomotive.

[Mark Saunders]

1. The straight air brake is just on the locomotive and is there just to apply the brake on the locomotive and used to hold a train stationary with the automatic brakes released prior st starting away. It takes time to release the automatic brake depending on length and distributor setting on the vehicles. Goods timing is slower to apply and release than passenger.

 

Air brakes that have triple valves on which was mainly emu’s with overlaid EP brake. The triple valve can have the brake application increased but as soon as the air pressure is raised it goes into full release.

 

3. A brake continuity test is to prove that the brake pipe is complete from the front of the train to the rear and that the brake can only be released by the drivers brake handle; the air or vacuum is destroyed and not being fed from anywhere else on the train! The brake can be applied from anywhere on the train but only released by the driver!

[Derekstuart]

Many thanks everyone. I recognise some of the terminology and it's this triple valve thingy that I remember being the reason why braking cannot be reduced without release. It's WHY that it that's the puzzler. But I think it will be in one of my Dad's old training manuals from when he was an Instructor- now that I know what to look for.

 

Many thanks again. A tremendous level of knowledge on this website.

Derek

[david.hill64]

Further to The Stationmaster's explanation about driving with the vacuum brake, the other reason for adopting this style for traditional stock with cast iron brake blocks acting on the wheels is that the friction coefficient between then iron block and the wheel rises rapidly as speed drops to zero. Although the force acting on the block is constant, the braking force will rise rapidly. This results in a significant jerk as the train stops, likely sufficient to cause problems for standing passengers. Stopping on a 

'rising brake' overcomes this problem.

[david.hill64]

4 hours ago, Derekstuart said:

Many thanks everyone. I recognise some of the terminology and it's this triple valve thingy that I remember being the reason why braking cannot be reduced without release. It's WHY that it that's the puzzler. But I think it will be in one of my Dad's old training manuals from when he was an Instructor- now that I know what to look for.

 

Many thanks again. A tremendous level of knowledge on this website.

Derek

The Wiki entry on Railway Air Brakes has a reasonable explanation of the workings of the Triple Valve.

David

[Jeremy Cumberland]

11 hours ago, Mark Saunders said:

The triple valve can have the brake application increased but as soon as the air pressure is raised it goes into full release.

I had forgotten this, and looked up my old diagrams from London Underground to refresh my memory. The dire warnings of not releasing the brakes fully between applications also came back to me.

 

Each carriage has an auxiliary brake reservoir, which is charged (via the triple valve) to the same pressure as the train pipe (65 psi for London Underground, but different railways used different pressures).

 

When the train pipe pressure is reduced, perhaps to 60 psi, the triple valve slides across and lets air from the auxiliary reservoir into the brake cylinder. This lowers the pressure in the auxiliary reservoir. When the pressure in the auxiliary reservoir is the same as the pressure in the train pipe, the triple valve moves back part way, stopping the flow. By this time, enough air has flowed into the brake cylinder to to create a pressure of, say, 15 psi in the brake cylinder.

 

Perhaps the driver increases the brake application, reducing the pressure in the train pipe to 55 psi. This lets more air from the auxiliary reservoir into the brake cylinder, perhaps increasing the brake cylinder pressure to 30 psi, and of course it reduces the auxiliary reservoir pressure to 55 psi.

 

As the train pipe pressure is reduced further, more air is let into the brake cylinder from the auxiliary reservoir, but there comes a point, somewhere around 50 psi, where the brake cylinder pressure equals the auxiliary reservoir pressure, and reducing the train pipe pressure beyond this has no additional effect. This is essentially an emergency brake application.

 

As soon as the train pipe pressure rises, the brakes release fully, discharging the air that had been in the brake cylinder to atmosphere. There is no such thing as a partial release; even if you only increase the train pipe pressure by 1 psi, the brakes will release fully.

 

It is technically possible for the driver to re-apply the brakes immediately, but now, instead of starting with an auxiliary reservoir pressure of 65 psi, you are starting with an auxiliary reservoir pressure of 50 psi (or whatever pressure the train pipe pressure had been reduced to). This in turn means that instead of being able to put 50 psi of air into the brake cylinders, the maximum you can now put in is about 35 psi, reducing the maximum available braking force. If you do a release and immediate reapply a second time, you will have even less air in the auxiliary reservoirs, and by now the brakes are severely compromised.

 

Whenever you release the brakes, you must ensure that the train pipe pressure returns to 65 psi (or whatever the standard is), and allow time for the carriage auxiliary reservoirs to fully recharge before making a second brake application.

 

[Edwin_m]

For long freight trains in North America, a brake release and recharge can take several minutes.  This is why separate locomotive brakes (generally dynamic, regenerating via the traction motors and dissipating heat in resistors) are so important for keeping the train under control when descending long gradients.  

[Steadfast]

Another use of the straight air is to control the train when shunting. 

 

If you're setting back at 3 mph and the speed is creeping up slightly, a quick rub with a couple of bar on the straight air will keep it in check. Applying the train brake you'd probably grind to a halt and have to get moving again. This use of the straight air allows very fine control of the speed when shunting, to the point of allow a heavy freight to be stopped very precisely. One train I've done using this method is stopped with accuracy of half a metre for discharge. 

 

Using the train brake in that situation would be a nightmare 

 

Jo

[The Stationmaster]

7 hours ago, david.hill64 said:

Further to The Stationmaster's explanation about driving with the vacuum brake, the other reason for adopting this style for traditional stock with cast iron brake blocks acting on the wheels is that the friction coefficient between then iron block and the wheel rises rapidly as speed drops to zero. Although the force acting on the block is constant, the braking force will rise rapidly. This results in a significant jerk as the train stops, likely sufficient to cause problems for standing passengers. Stopping on a 

'rising brake' overcomes this problem.

A big handling problem with traditional cast iron brake blocks is that their condition can affect brake performance quite noticeably.  Worn blocks are an obvious potential problem area but new blocks on a complete train can be just as bad and they need a number of brake applications to get the best performance.

 

The WR London commuter sets were always re-blocked in total by Old Oak C&W ona PPM basis so you had a complete train with new blocks and you had to adjust your braking point/technique especially if you only usually braked with a single application and then released the brake.   On a train with new blocks getting a double yellow at Burnham on the Up Main was ideal as you then had a chance to get the brake rubbing before you needed to make a normal service application if you got a single yellow at the next signal.   But once the blocks were at peak performance a vacuum brake on a passenger train was a delight to use in my experience - and i used to drive regularly in the days when i was a depot manager.

[DY444]

On 10/04/2023 at 07:01, Jeremy C said:

As far as I am aware, British Rail never used triple valves on loco hauled stock, using distributors instead (which can be progressively released), but London Underground did, and I imagine the Great Eastern did too, and the Southern and its constituents for their electric trains.

 

Not loco hauled stock in the normal sense but the SR 4TCs had triple valves and so when hauled by, or worked in push-pull formations with, locomotives other than 73s (which had EP brakes), there was no graduated release available. 

Edited April 11, 2023 by DY444

[Derekstuart]

10 hours ago, Jeremy C said:

Whenever you release the brakes, you must ensure that the train pipe pressure returns to 65 psi (or whatever the standard is), and allow time for the carriage auxiliary reservoirs to fully recharge before making a second brake application.

 

 

1 hour ago, DY444 said:

Not loco hauled stock in the normal sense but the SR 4TCs had triple valves and so when hauled by, or worked in push-pull formations with, locomotives other than 73s (which had EP brakes), there was no graduated release available. 

 

Now that you write this, I seem to recall being told something similar/ same in relation to the Cannon Street crash some years back. The Driver had apparently made several brake applications (which could be done on one class but not on the one that crashed, IIRC).

Thanks again everyone. That's explained it all perfectly and I'm sure others will go on to read and understand as a result.

PS I am a fully qualified commercial vehicle mechanic and our air brake systems are much more simplified, as you'd expect. Foot brake needs air pressure to apply the brakes and park brake needs air pressure to release them. Aside from anti-compounding valves and relays- which themselves are straight forward anyway- the system really is as straight forward as it seems, which you need when you're constantly swapping one pedal for the other on a busy road.

[BR traction instructor]

Even in the 1980s when I joined BR as a Traction Trainee and spent over 2 years as a Secondman before progressing to Driver, there was a myriad of brake systems in use, with even greater complexity potential in out of course situations. One of the most challenging braking situations occurring whilst running light loco with a class 47/3 from Buxton to Guide Bridge to collect a rake of wooden bodied opens to take to Ditton sleeper depot. Approaching the junction at Hyde we stopped at a red aspect and were advised by the signaller via the SPT that a class 303 EMU on a Glossop to Manchester Piccadilly commuter service had failed at Flowery Field. We were asked to haul it to Piccadilly, stopping at all stations. As a secondman driving under the supervision of my driver that day I carefully obeyed the signaller’s instructions to pass a signal at danger, reverse (change ends) beyond the junction and proceed slowly wrong line onto the front of the failed unit, stopping to pick up the guard with his red flag. My driver chuckling at the braking challenge to come. A movements supervisor had been to Longsight depot to collect the brake pipe extensions necessary to couple our 47 to the 303. My driver instructing me on the vagaries of hauling Triple valve fitted stock with a loco. Treat the train brake gently at each station he cautioned, the brake will need to be completely released before it can be reapplied. Once we were coupled up (buckeye dropped & buffers extended/saddles inserted) and brake tested a green flag from the guard and proceed aspects towards Guide Bridge allowed us to get under way. Approaching the first stop at Guide Bridge with an unknown brake was interesting…putting too much in and risking stopping short the most likely outcome. Listening to my old hand driver’s coaching and using a light application as a running brake test to get the feel of the brake before each station stop paid dividends and at least made for getting the 303 completely on the platform, even if the stop was a little sudden. Each station after that was a further opportunity to practice. Arriving in the terminus at Piccadilly the dead 303 (pantograph carbon cracked, ADD operated) was removed by another EMU to Longsight, allowing us to return to Guide Bridge to collect our sleeper wagons for Ditton.

 

BeRTIe

Edited April 14, 2023 by BR traction instructor

[roythebus1]

The TC units had the standard SR EP brake and triple valve air brake. 

 

On a train with triple valves the driver only had 3 brake applications before he ran out of air.

 

To quote "Steadfast" from above, 

"If you're setting back at 3 mph and the speed is creeping up slightly, a quick rub with a couple of bar on the straight air will keep it in check. Applying the train brake you'd probably grind to a halt and have to get moving again. This use of the straight air allows very fine control of the speed when shunting, to the point of allow a heavy freight to be stopped very precisely. One train I've done using this method is stopped with accuracy of half a metre for discharge."

 

Amateur!! :) On the Waterloo& City with the old Bulleid stock it only had the Westinghouse brake. A good driver on there could stop at Bank with the centre buffer plate  bouncing forward, kissing the buffer stop, bouncing back, and not tripping the "sleeping policeman" warning that the train hit the block. I was doing a film job there one Sunday, Dempsey and Makepeace, and the director wanted the train to stop at a white line on the platform (our S mark). I backed out of the station, ran in and stopped by the line. He asked if I could do it again, I asked what side of the line he wanted me to stop at. Having done it 16 times a day, I think I'd mastered the Westinghouse brake.

 

The Isle of Wight, Ravenglass & Eskdale and a few other narrow gauge lines use the Westinghouse brake.

 

LT Railways used to run Westinghouse at 65 psi, the mainline used 70psi, but that changed in 1978 to the European standard of 72.5 psi or 5 bar.

 

[Jeremy Cumberland]

1 hour ago, roythebus1 said:

The Isle of Wight, Ravenglass & Eskdale and a few other narrow gauge lines use the Westinghouse brake.

 

The Ravenglass and Eskdale uses single pipe air brakes, but it doesn't use triple valves, and brakes can be partially released.

 

12 hours ago, Derekstuart said:

Now that you write this, I seem to recall being told something similar/ same in relation to the Cannon Street crash some years back. The Driver had apparently made several brake applications (which could be done on one class but not on the one that crashed, IIRC).

I took a look at the Cannon Street accident report. The train was composed of 4-EPB and 2-EPB stock, with EP + Westinghouse brakes as was standard on the Southern Region at the time. An off-duty driver riding in the cab noted nothing unusual about the driver's driving, although he had not been paying much attention to it, but there was no question of the train running out of air since the brakes need to be held off on the approach curve to Cannon Street otherwise power would need to be applied again  to get into the platform. The driver appears not to have made a brake application at all, not until the last second when he put the handle into emergency, and the general conclusion was that the driver's attention lapsed as he came into the platform. The driver did say to an off-duty driver in the cab, just before the crash, that he had lost the EP brake, but even an EP brake failure should not have resulted in a collision at that speed (about 10 mph). An inspection of the train in situ shortly afterwards showed the brakes were applied down the train, with  brake cylinder pressures of about 50 psi. as would be expected.

[Steadfast]

3 hours ago, roythebus1 said:

Amateur!! :) On the Waterloo& City with the old Bulleid stock it only had the Westinghouse brake. A good driver on there could stop at Bank with the centre buffer plate  bouncing forward, kissing the buffer stop, bouncing back, and not tripping the "sleeping policeman" warning that the train hit the block. I was doing a film job there one Sunday, Dempsey and Makepeace, and the director wanted the train to stop at a white line on the platform (our S mark). I backed out of the station, ran in and stopped by the line. He asked if I could do it again, I asked what side of the line he wanted me to stop at. Having done it 16 times a day, I think I'd mastered the Westinghouse brake.

Very much so... I only passed out for driving last week, so if I can do it, anyone can 😋

 

That's impressively accurate stopping on the W&C. When you see the experienced hands handling a train with ease, that's something to aspire to in my book.

 

Regarding triple valves, apparently lots of continental ferry wagons still have them. I've never seen one in my years of shunting, they're that rare on the modern wagons where everything has a distributor, but we're trained that trains of ferry vans should be treated as triple valves unless known to be distributor fitted.

 

Jo

[pheaton]

On 09/04/2023 at 22:12, Derekstuart said:

Hello

Could I ask any Driver/ Guard/ Engineer/ knowledgeable person a few questions that have been puzzling me, please?

1. Why have a separate locomotive brake control when, as I understand it, the train brake control will work fine when there's no train connected. I'm sure there's a good reason.

 

2. Why- at least on some classes- can you not reduce braking and instead have to release the brake and then re-apply to the appropriate level.

 

3. When coupling a locomotive to stock, I know you have to test the brakes. As I understand it this is applying the loco brake (see 1?) or hand brake, releasing the train brake then someone opening the train brake (red?) cock at the other end to see if air comes out and in doing so the Driver/ Guard notes the TB pressure dropping. Is that correct or is there anything else?

 

Many thanks in advance for this question of idle curiosity; I'm sure there's not only a good reason for 1 and 2.

 

Derek

 

From a BR first generation diesel point of view (up to and not including class 56 and specifically excluding "the twins" and the other oddball locomotives at the time..).

 

As you probably know already all the first generation diesels dual braked or not had 2 brake handles 

 

Straight AIR Brake (which operates the locomotive brakes only) 

 

Train Brake (which operates the train brake primarily and depending on the setup the locomotive brake proportionally)

 

Proportional?????

 

What do we mean by proportional....we mean that a part of the braking effort is shared by the locomotive.....the amount of braking effort depends on 2 things...

 

1 the timing in use (is it goods or passenger)

2 the valve types in use on the brake frame (this primarily in my knowledge applies to Davis Metcalfe fitted locomotives

 

Without a train attached the locomotive is controlled by the straight air brake....effort will be instantly applied to wheel-sets on the locomotive.. the driver is able to precisely control the amount of effort on offer to a pre-described amount (58 to 60 psi)

 

You can use the train brake without the straight air brake to stop a light locomotive but....there are several things to consider....

 

1,) The braking effort on the wheels will be considerably delayed until either the vac pipe has dropped to a predetermined level or...the ABP has dropped to a predetermined level. Therefore your not going to stop as quick...

 

2,) The braking effort on the wheels will be considerably reduced (will explain later) as its applied proportionally to the level of braking requested in either the vac pipe or the ABP.  (this gets more complicated later)

 

3,)  locomotive power will be lost completely below 15" on the vacuum brake pipe and below a certain level on the ABP....because why would you want to power against the train brake?? You can power to your hearts content against the locomotive brake...for reason as mentioned by our resident train drivers above....such as buffering on creeping forward etc etc...

 

So in a nutshell....if you were to operate the train brake on a vacuum brake train (assume fully fitted) the brakes on the train do most of the work, the brakes on the train come on first. \

 

To give you an idea.. in vacuum

 

21" = full release (no brakes) You would have this prior to departing a station or a signal stop and the train would be held by the operation and release of the locomotive straight air brake....(why its a lot quicker than a vacuum brake)

 

below 21 and above 15" = a light brake application on the carriages and.....about 5- 10 PSI in the locomotives brakes (again depending on the timing and the valves in use on the brake frame but thats another chapter)

 

15" to zero....more brake on carriages more brake on the loco....in an ever increasing proportion until you reach 0 int he vac brake pipe and on some classes over 60 psi in the loco......in other words....drop anchor!!!

 

Air works in the exactly the same...way but the value will be in PSI and theres no vacuum involved....i could explain in detail but i think you all get the picture....instead of reducing vacuum...you reduce the air brake pipe and again a proportional amount of braking is applied to the locomotive.

 

could you stop a train on the straight air brake only.....yes you can but....

 

1,) if you have a passenger train your passengers will end up squashed against the bulkheads because the locomotive will stop before the carriages do...the carriages will go into the locomotive and the obviously laws of physics apply to the contents...this especially applicable to screw link vehicles.....

 

2,) as the train buffers up against the locomotive it will stop forcing the loco forward....doesn't do your wheel-sets any good and then the train will concertina until all the inertia is gone...or derail....either way...its going to be a mess inside....

 

3,) your braking distance will be exponentially increased....this is why light locomotives always have a speed restriction applied!

 

the idea of the proportional brake is that the locomotive stops at exactly the same time as the carriages...nice and smooth....

 

the bonus complicated bit.....

 

NO Dual BRAKED or VAC ONLY LOCOMOTIVES (excluding the twins or the other oddball types around the time) operate the mechanical side of the locomotives brakes using Vacuum even if....your are hauling a vacuum train it is always air.

 

WHAT?????????

 

Thats right...the mechanical bit....ie the brake cylinders are exclusively air. Vacuum does not directly operate any mechanical braking systems on the locomotive, it only influences it.

 

This is because as mentioned a diesel loco (excluding the twins and the other oddball types) does not have a vacuum cylinder...unlike a wagon or a carriage or a steam....or the twins or the other oddball types.

 

Every dual braked locomotive has 4 brake gauges

 

1, ABP (air or automatic brake pipe)

2 VAC Chamber/Vac Pipe (normally a duplex gauge)

3, Bogie brake Gauge

4, Main Res pipe gauge (not all locos have this though)

also Main reservoir gauge (not really a brake gauge but....included for complete ness)

 

In vacuum on a dual braked locomotive when you operate the train brake valve in vacuum you are influencing the amount of pressure in the Automatic brake pipe. as this pressure reduces a valve in the brake frame will reduce vacuum accordingly (by a predetermined level) by gradually venting the vacuum pipe to atmosphere this then reduces the vacuum in the pipe on the train and causes the vacuum brake cylinders to apply the brakes accordingly. Additionally in the locomotive and as mentioned above there is also a vacuum chamber, and this is simply a tank about the size of a fire extinguisher which is constantly kept (regardless of braking) at 21" this is there as a reference....a second set of valves on the brake frame then compare the value of the brake pipe with the value of the chamber, and the difference is translated to a proportional application of the locomotives straight air brake automatically by admitting air in defined amounts to the brake cylinders. The timing switch (goods/pass) allows this process to be completed by different relay valves which have chokes in and that determines the amount of proportionality applied.

 

If the level of vacuum in the chamber falls the braking proportionality will be reduced, if it falls to zero the locomotive will no longer have a reference and therefore will never apply its brakes.

 

The only influence the train brake valve in a dual brakes locomotive has on vacuum is in emergency when a mechanical flap opens on the brake pipe which is operated by pushing the handle to emergency which immediately vents the brake pipe to atmosphere instantly reducing it to zero and applying the brakes in full as described above. As the brake pipe is vented by a large opening the exhausters cannot overcome this and will never be able to create more than 1 or 2 inches of vacuum.

 

The same effect would be achieved by the train parting!

 

On a vac only locomotive there is NO ABP (it doesn't have air train braking) so instead of the valves associated with the air brake pipe telling the valve to vent the vac brake pipe, the venting of the vac pipe is controlled by the train brake handle itself, after that the principles for applying the proportional locomotive air brake are the same.

 

most dual braked electric locomotives work exactly the same way...but i'm not sure of the wood-head locos i believe they are the same as a vac braked diesel

 

If anyones interested i could quite happily do a video of it in operation this weekend.

 

 

 

 

 

 

 

 

 

 

 

 

[roythebus1]

Not forgetting that we used to run a lot of goods trains that only had handbrakes. they relied purely on the loco air brake (or steam brake) and the guard in his van at the back of the train giving braking assistance by turning the big wheel in his brake van.

 

The first time I was let loose driving a BR train was a class 25 and about 600 tons of coal on the Southam branch from Rugby. Having been a guard on the underground previously I was used to driving electric trains with brakes. So doing 20 along the branch I braked for dropping wagons off at Bilton Cement Works. I'd say it took a very long way to stop 600 tons using the loco brake. It was the ideal place to teach someone what an unfitted train was like to drive. It was 'our" bit of railway to do what we liked on. One-engine-in-steam and all that.