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The concept is simple enough: no pressure in the train line has the brakes locked, full pressure is loose, and partial pressure drags accordingly. So if pressure is lost for any reason, the train stops. (or at least it's supposed to; more on that later) So I'd expect to see the brakes applied by a spring that is capable of stopping the wheels of a fully loaded car, and a single-acting cylinder that works against that spring to release the brakes. A hand-brake would also work against the spring, in parallel with the air-cylinder.

But everything I've seen to explain how it actually works includes the same seemingly unnecessary complexity, as just a matter of fact with no mention whatsoever about why. They do, however, mention the pitfalls of this overly complex system and how it's caused accidents.

Likewise, every car that I've had access to crawl under seems to confirm the explanations. The obvious direction of forces (thin pull rods for a direct giveaway) requires positive pressure to apply, not positive pressure to release.

The way that it actually works, has a local reservoir on each car that is "charged" by the train line. When the train line reduces pressure, a purpose-designed valve allows a corresponding amount of pressure from the local reservoir into the brake cylinder. This positive pressure applies the brakes.

There is also an emergency function that senses a sudden drop in pressure and finishes dumping the train line at that car. Then the next car sees a sudden drop, etc., thus propagating an emergency brake signal much faster than would otherwise be possible.

The obvious pitfall that has indeed caused accidents, is that it takes time to recharge all of those local reservoirs. So if you've just released the brakes, the cylinders are dumped quickly, but it's impossible to reapply them for a while, at least to full effect, because the local reservoirs haven't recovered yet. Then I see more complexity being added, in the form of a second train line that is always "charged", for the purpose of keeping the local reservoirs topped off...

The only reason I can think of to not use a direct system, with a spring to apply and direct air pressure to release, is to reduce the time from completely-locked to completely-loose, as the local brake cylinder can be dumped to atmosphere effectively instantly. The emergency function wouldn't be affected, since it can dump the cylinder to stop in the same way that it presently dumps the cylinder to go. And there are "fast-return" valves for "ordinary" pneumatic actuators that dump the difference in pressure to the atmosphere, which in this application would allow the brakes to be applied with no more delay than they are presently. (the train line still only needs to discharge its own volume, not the cylinders') At this point, the only remaining difference is that it takes longer to release the brakes, as the train line is filling the cylinders instead of reservoirs.

It may sound like I'm adding the same level of complexity as what the present system has, but I think a key difference is that none of it is absolutely required for the system to work at all. It's only an enhancement of what's already there, compared to a fundamental change in operation if part of the present system is removed.

Is it really worth the complexity and risk to have a train leave that quickly? Or is there something else that I'm still missing? To use some electronics terms, why is it necessary to reverse the polarity of the control signal on each car and rely on a small amount of stored energy that is slow to replace, instead of using an actuator that takes the supplied polarity directly?

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    $\begingroup$ @AaronD, Although, this question appears to be a well documented one, I am having trouble following, simply because there are many implementations of train brake systems out there. IMHO, it would be helpful if you provided some schematics/figures with the systems you are considering, so that your question can be better answered. $\endgroup$
    – NMech ♦
    Jan 21, 2022 at 10:07

4 Answers 4

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You need to consider what happens if a hose bursts, and your spring-applied brake locks the wheels on one or more cars - how do you get that train back to the depot? There could be a manual brake release lever, but this would have to be on the brake cylinder, and there are many circumstances where crawling under the train to access this lever would be difficult or impossible.

On many trains, and especially on mass-transit vehicles, the crew need a safe & easy way of isolating and releasing the brakes on one car, that is accessible from within the passenger compartment. The answer is to have air-applied brakes, so by turning a couple of valves the brakes can be isolated & released, and the train can be moved out of service without too much delay - time is of the essence, in a mass-transit breakdown.

However, some London Underground stock (e.g. D78) had spring-applied brakes as you describe, but these only apply sufficient braking force to hold the train static, i.e. they are a parking brake. This level of force is chosen so that in an emergency, if the parking brake is locked on, the train can still be moved, albeit with a much-increased rolling resistance.

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  • $\begingroup$ The hand brake control doesn't have to be on the cylinder itself. I said in the OP that it could work in parallel with the air cylinder, but that does not mean that the control has to be there. (there are such things as cables and push rods...) Regardless of air pressure, this could override the spring by itself, which is enough to solve the problem that (I think) you describe. $\endgroup$
    – AaronD
    Jan 21, 2022 at 20:08
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    $\begingroup$ Is it really worth the complexity and risk to have a train leave that quickly? Yes. $\endgroup$
    – Mazura
    Jan 22, 2022 at 3:47
  • $\begingroup$ Every cable or push-rod you add is a potential failure-point, a safety-risk if it gets jammed in the wrong position, and a maintenance liability in terms of inspection, lubrication, and replacement of worn-out parts (the bogie is a very high-vibration environment). Once car might have 2 wheelsets per bogie, 2 bogies per car, and one brake cylinder per wheel, so you'd be adding a lot of extra gear. $\endgroup$
    – jayben
    Jan 22, 2022 at 9:49
  • $\begingroup$ @jayben The cars that I've crawled under have a single brake cylinder and a complex system of pull rods and levers to transmit that force to every wheel on both bogies. Complex enough to require a diagram affixed to the car to explain it, which was still clear after the car was decommissioned and sat for at least a few years as a display piece. That system could be modified to pull a bunch of individual springs away from the wheels, or kept as-is with a single spring/cylinder combo in place of the existing cylinder. So those potential failure points already exist, and are apparently okay. $\endgroup$
    – AaronD
    Jan 22, 2022 at 17:54
  • $\begingroup$ It seems to be a recurring theme, that flexible pressure hoses are a massive liability to be avoided whenever possible, even if it creates a complex mechanical linkage. Hard pipes are fine, but nothing flexible unless it's absolutely required and trivial to get to, like the coupling between cars. And I don't think it's all that expensive in this setting, to over-build a mechanical link so that it practically never fails. Thus, the single cylinder driving a mechanical mess. $\endgroup$
    – AaronD
    Jan 22, 2022 at 18:00
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This answer addresses the following sentence:

So I'd expect to see the brakes applied by a spring that is capable of stopping the wheels of a fully loaded car, and a single-acting cylinder that works against that spring to release the brakes.

IMHO, one very basic reason against using the spring is the fatigue and the degradation of the elastic properties of the materials. I.e. when the spring is repeatedly loaded and unloaded fatigue can become a cause of concern. In order to circumvent that you'd need to go several times over the safety factor of the application, which would mean weight and cost.

On the other hand, the main cost with air systems is generating the pressure and distributing it. And since in trains there is already an compressed air line, the cost of making the whole brake system working with air is preferable. Additionally, a pneumatic system has the added benefits of significantly higher loads compared to a spring, and also in many ways it's easier to conduct inspection of it.

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  • $\begingroup$ I'm not removing the air system. Nor am I even changing the control end at all. I'm just (conceptually) simplifying the cars, using the exact same control: compressed air into the car to release, remove pressure to apply. Your mention of fatigue is certainly valid though. The brake spring would have to be included in the regular inspection and considered a wear item, similar to the suspension springs. $\endgroup$
    – AaronD
    Jan 21, 2022 at 20:37
  • $\begingroup$ @AaronD (maybe I wan't clear enough) however I replied thinking that the air system would be still there for the single-acting cylinder that works against that spring to release the brakes". And the presence of that part is what makes air a prime candidate for the brake application part. $\endgroup$
    – NMech ♦
    Jan 22, 2022 at 14:34
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  1. With an "spring apply / air release" system the spring pressure may not be enough to apply adequate braking force. Using compressed air to apply the brake allows much higher braking forces.

  2. The reservoir system when discharged allows rough shunting of the cars without the trouble and delay of having to couple the cars completely.

  3. Hump shunting wouldn't be possible.

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  • $\begingroup$ I think in general you want the cars to roll freely unless "instructed" to do otherwise, for example by tightening hand brakes, not only during "hump shunting", at least for detachable cargo cars. $\endgroup$
    – Peter - Reinstate Monica
    Jan 21, 2022 at 18:02
  • $\begingroup$ #1 is irrelevant. You can make both a spring and an air cylinder with any strength you want. #2 and #3 are good points though. My version would require the hand brake to be used on every car, or perhaps the air line sealed with sufficient pressure still in it, to make that work. Still possible, but a different mode of operation that has its own set of challenges. $\endgroup$
    – AaronD
    Jan 21, 2022 at 20:31
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    $\begingroup$ @AaronD, I disagree (but I haven't done any calculations). You can make the spring as strong as you want but then have to size the pneumatic release mechanism to match and you have to pay for both. As far as I know the common air brake systems don't use springs at all. $\endgroup$
    – Transistor
    Jan 21, 2022 at 20:48
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    $\begingroup$ @Transistor The pneumatic release would be the same size as today's pneumatic apply. It's the same force, after all, derived from the same pressure. Maybe only slightly bigger to guarantee that it does release, but not by much. Or...maybe it would be smaller because the currently-used local reservoir ends up with greatly reduced pressure by the time you get a decent application. So the currently-used application cylinder needs to be bigger to get the required force from that reduced pressure. $\endgroup$
    – AaronD
    Jan 21, 2022 at 21:02
  • $\begingroup$ @Transistor You do have a good point though, about adding a spring that didn't exist at all before. I wonder how it compares to the reservoir that would go away, and a possibly different-sized cylinder? $\endgroup$
    – AaronD
    Jan 21, 2022 at 21:02
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Because the system has worked for a long time and railroads don't want to spend money on improving a technology is time tested. Most railroads run locomotives that is over 25-50 years old. (Most north american railroads still have a fleet of EMD GP-40's that were manufactured in the late 60's to 70's). Most railroads cut corners to save a dime so I don't see them changing this any time soon.

https://en.wikipedia.org/wiki/EMD_GP40.

Why would you replace something that works almost perfectly?

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  • $\begingroup$ The locomotives still work exactly the same way. In fact, the air-reduction system was designed for steam, long before anything else existed. (steam-powered air compressor because air doesn't condense in the train line like steam would; the only change required for modern trains is how to get the compressed air to run an otherwise unchanged system) $\endgroup$
    – AaronD
    Jan 21, 2022 at 20:12
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    $\begingroup$ The original braking system had the naive positive-polarity control scheme where the braking pressure was supplied directly by the loco, so if you lost that pressure you lost your brakes entirely. Air reduction was a rather quick correction to that, as I understand! $\endgroup$
    – AaronD
    Jan 21, 2022 at 20:13
  • $\begingroup$ Germany's Deutsche Bahn also has lots of engines from the 70s to 90s, e.g. BR 111, BR 143, BR 218, BR 232. These are gradually being replaced though. $\endgroup$
    – Jan
    Jan 21, 2022 at 20:20
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    $\begingroup$ A different reason is because if you were to implement a new system the new cars would not be compatable with the old cars. This would require railroads to re-do their entire fleet. What if one railroad refuses to upgrade? It would mean that the new car would have to be unloaded in to an old car anding time to the trip & requiring more workers and more expense. Also some shortlines may not have the money to buy new or upgrade locomotives. While it may be complicated it is far cheaper than replacing an entire fleet of locomotives and cars. $\endgroup$
    – benjm
    Jan 22, 2022 at 15:47
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    $\begingroup$ @benjm The interface to the car is exactly the same. Same connector, same working pressure, same effect caused by changing that pressure. Only the mechanics of the car itself would be different. So no incompatibility at all. $\endgroup$
    – AaronD
    Jan 22, 2022 at 17:42

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