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Scenario:
Suppose the emergency brake button in the cab of a train is not working. Would the emergency brake cords / buttons in the passenger cars still work?

This question is inspired by this question on the SciFi StackExchange, asking why the passengers in the train fight scene from the movie SpiderMan 2 did not simply pull the emergency brakes.

In that particular scene, a R46 City Subway Car was rendered "unstoppable" by ripping out the speed control lever in the car, which also happened to disable the emergency brake button (see 16 seconds into the video).


With movies, we suspend our disbelief in order to enjoy the story that is presented. But the above SciFi question got me to thinking about how emergency brake systems are designed for trains.

Given trains significant mass and momentum when moving, it seems like there would be multiple, redundant safety systems to provide braking capability for the train.

My Question:
Is there a common safety design used for the braking system of trains?

Does that design account for portions of the system failing and allowing other portions to compensate for the failed components? (ie. would the passenger car emergency brakes still work?)

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    $\begingroup$ As far as I know in UK trains, the "emergency" cords/buttons in passenger cars don't actually directly apply any braking mechanism, they inform the driver/guard and possibly the network operator and they decide what to do. Usually the procedure is to stop only if any part of the train is still in a station, otherwise continue to the next station and stop there. $\endgroup$ – jhabbott Mar 18 '15 at 20:38
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Train Brakes

The common brakes on trains are air brakes. As the name implies, these work off of air pressure. The braking power isn't controlled in the way that you would immediately think of though. They do not work like car brakes where the harder you press on the brake pedal, the harder the pressure goes through the lines to the brake cylinders. They work the opposite. The less pressure in the line, the more braking force is applied.

Fail-safe

Rail brakes are designed to be fail-safe. That is, when a failure occurs, the safe operation happens. In a train, all of the cars have air lines that are attached together. This forms one long, continuous line from the locomotives through all of the other cars.

This air line is used to both fill the air reservoirs on each can and control the amount of braking. This is accomplished through an ingenious valve that works solely on air pressure. As mentioned above, once all of the reservoirs on each of the cars has been filled, a decrease in the air pressure in the air line causes the brakes to be engaged.

The beauty of this system is that is something bad happens and the air pressure drops, the brakes are engaged automatically. This means that if there is a rupture in the air line, or cars become uncoupled, the brakes are fully engaged without any other action.

Actually, if there is very low or no pressure in the line, a specially amount of braking force is applied called emergency braking.

Manual Brakes

The emergency braking force is enough to stop the train or a single run away car, but due to leakages, the braking force will eventually reduce. This is why train cars also have a mechanical brake. It is engaged by turning the big brake wheel are the end of the car. These are also the brakes that must be engaged to keep cars from rolling away if they are left alone in a siding.

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    $\begingroup$ The emergency brake (triggered by a rapid drop in pressure, not no pressure) also draws on its own separate reservoir in addition to the normal reservoir (so even if the service reservoir was spent by repeated brake application, it'll still be able to stop). $\endgroup$ – cpast Mar 19 '15 at 6:10
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    $\begingroup$ Incidentally, the use of automatic brakes (fail-safe) in the UK was mandated as a result of the 1889 Armagh rail disaster en.wikipedia.org/wiki/Armagh_rail_disaster A train stalled on a hill and the carriages were manually separated, the brakes failed and they rolled away colliding with another train, 80 dead, 260 injured. $\endgroup$ – xcxc Mar 19 '15 at 6:39
  • $\begingroup$ @xcxc Perhaps Canada could learn from such. $\endgroup$ – Cees Timmerman Mar 19 '15 at 9:51
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    $\begingroup$ @CeesTimmerman While the immediate effect of a loss in air pressure in the line is to stop the train, the train brake still relies on air pressure from the locomotive (it's just that that builds up when the train's operating normally, and is applied when the line has a drop in pressure). When the cars are separated from the locomotive for enough time, the air pressure becomes not enough to hold them, so you need the hand brakes. The point of automatic brakes is to stop the cars, not to keep them stopped forever (once they're stopped, staff can apply all the hand brakes). $\endgroup$ – cpast Mar 19 '15 at 15:02
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    $\begingroup$ @PatFromCanada Unless we are talking about different things, the first link in the answer disagrees with you. $\endgroup$ – hazzey Mar 19 '15 at 18:02
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Short answer: I suspect that ripping out the speed control lever would have immediately applied the emergency brake.

In every safety system I have ever designed, emergency stop buttons are normally closed switches so that nothing will operate unless the switch conducts current. Therefore if the wires to the emergency stop were severed, the brakes would come on. The brakes would also come on if the brake vacuum pump was damaged, because the air brakes in trucks and trains will be on unless the engine is actively turning them off.

Note: I have never worked on train brakes, I'm basing my answer off of my personal knowledge of truck brakes and aircraft flight controls.

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I was trained to operate diesel electric locomotives for the Canadian Pacific Railway about 20 years ago. I'm not sure if the technology has been updated, likely not that much. The other posts about how the brakes work are correct. Air is pumped from the locomotives to fill reservoirs on each car to be able to release the brakes. Any large loss of pressure along the train applies the brakes. From the locomotive there are 3 ways to initiate emergency breaking. Press the engineer emergency brake button or let the timer expire on the engineer emergency brake button. (setup so if something happens to the engineer it will automatically stop the train). There is a also an emergency brake lever for the conductor to pull on his side of the cabin. The last way is change the direction of the throttle form forward to reverse or reverse to forward. All of these will release the air pressure and cause all the brakes to be applied both the train car brakes and locomotive brakes.

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  • $\begingroup$ If I'm not mistaken there is another way to initiate emergency braking: the Alerter, or dead man device. On most US trains, as I understand it, if you do nothing for a period, maybe 30 seconds, it will go into emergency braking. $\endgroup$ – DGM Mar 19 '15 at 14:10
  • $\begingroup$ Maybe it isn't standard, but I thought that the FRED could also be remotely controlled to dump the air. Maybe it is automatic when an emergency is initiated from the locomotive anyway. $\endgroup$ – hazzey Mar 19 '15 at 14:15
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The braking system in a train is designed so that the loss of pressure at any point along the trainline will trigger the brakes -- an end-of-train-device, distributed-power locomotive, cab car/cabbage, or a passenger car emergency brake handle can still operate the brakes (service and emergency, or emergency only, depending on what's operating it) even if the head-end's brake valve is inoperable for some reason (say, a mechanical jam). Furthermore, the brake valve lever on a road-going locomotive in the US (whether it uses a control console or a traditional control stand) is mechanically separate from the throttle (speed control) and reverser levers -- you can still apply the brakes if the reverser is pulled or if the throttle is jammed.

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