Question about the brake system on trains

If the air is lost in the brake system, the brakes are fully applied in what is called an "emergency" application. LOWERING the air pressure in the brake pipe is what APPLIES the brakes. It is a fail-safe system.

We don't yet know what caused this accident, but I'll guarantee it was not because the brakes failed.

OGR Webmaster posted:

If the air is lost in the brake system, the brakes are fully applied in what is called an "emergency" application. LOWERING the air pressure in the brake pipe is what APPLIES the brakes. It is a fail-safe system.

We don't yet know what caused this accident, but I'll guarantee it was not because the brakes failed.

Thanks, Rich, that is what I recalled about them. I remembered that Westinghouse when he designed the original system had it as the air actuating the brakes (kind of like on a hydraulic system), but realized that would have failed had there been a leak, so he reversed it, sounds like that, with modifications, is what is used today. 

I doubted it was brake failure as well, I kind of wonder from the Engineer not remembering (which also could have been induced by head trauma from the accident), if he had some sort of petite mal seizure. Other than that, this is a head scratcher. 

George Westinghouse's system originally tried at the Burlington tests was a system which, like today, reduced pressure in the brake pipe to set the brakes. If failed because it worked so slowly that there was damaging slack action in the train.  When confronted with this George immediately knew what to do as he had already invented the air signal.  It depends on wave propagation rather than pressure drop to operate the air whistles. George immediately redesigned the air brake system to use wave propagation.  The system was retested by the Burlington and was a complete success.  This is the bases of the systems used in North America today and on most air brake systems around the world.

George was the first president of the American Society of Mechanical Engineers. When he was about to step down from this position he was ask to write a paper on how he invented the air brake system.  It is a very well written paper and contains a great deal of technical information on air brakes. I have always had a feeling that most of his staff had a lot of input in to this paper.  After the paper was presented at the ASME convention's annual dinner,around 1890, Westinghouse Air Brake Company reprinted and distributed many copies of the paper.  So many, in fact, that it is not hard to find copies in used book stores today.   Very interesting to read, but it does get very detailed in the features of the various systems and the changes made to them and why.

Air brake failures are rare, but they do happen.  I think most are associated with failing to properly setup, charge, and test the system initially, frequent applications and releases which do not allow for complete recharge of the auxiliary reservoir, excessive leakage, or a blocked brake pipe.   On this recent accident I would not expect any of these issues since this was a final terminal and the air brake system must have been working. In a modern transit application, there is probably a basic Westinghouse style air brake system with an electronic control system overlay and main or control reservoir air  trainlined to allow fast applications and releases at each car as commanded by a microprocessor reading commands from the cab. 

OGR Webmaster posted:

If the air is lost in the brake system, the brakes are fully applied in what is called an "emergency" application. LOWERING the air pressure in the brake pipe is what APPLIES the brakes. It is a fail-safe system.

We don't yet know what caused this accident, but I'll guarantee it was not because the brakes failed.

Rich,

When you reduce the brake pipe pressure to apply the brakes, and then increase the pressure to release the brakes, the air that was used to actually apply the brakes comes from a reserve tank on each car? then is released to the atmosphere?  If that is the case, what happens after several application/release cycles?  Is there a danger of not having enough pressure to apply the brakes Trying to understand the operation.

Larry

LLKJR posted:

OGR Webmaster posted:

If the air is lost in the brake system, the brakes are fully applied in what is called an "emergency" application. LOWERING the air pressure in the brake pipe is what APPLIES the brakes. It is a fail-safe system.

We don't yet know what caused this accident, but I'll guarantee it was not because the brakes failed.

Rich,

When you reduce the brake pipe pressure to apply the brakes, and then increase the pressure to release the brakes, the air that was used to actually apply the brakes comes from a reserve tank on each car? then is released to the atmosphere?  If that is the case, what happens after several application/release cycles?  Is there a danger of not having enough pressure to apply the brakes Trying to understand the operation.

Larry

Yes, an engineer can "use up the air" if too many brake applications are made without sufficient time to recharge the train line in between applications. This is a real danger with heavy trains on long downgrades, especially before dynamic braking came into common use. I talked about this in some detail in a previous post on this thread and it was deleted, why? It's all part of the discussion for people who are interested in brake systems.

https://en.wikipedia.org/wiki/...ir_brake#Limitations

I am not Rich, but I can answer your question.  In its simplest form the railway brake system, on each car, has a triple valve, a brake cylinder, auxiliary reservoir and the brake pipe.  The triple valve has three functions to preform 1) to charge the auxiliary reservoir to the brake pipe pressure through a choke and check valve, 2) when the brake pipe pressure is reduced the triple valve routes air from the auxiliary reservoir to the brake cylinder keeping the auxiliary reservoir and brake pipe pressures at the same pressure, and 3) releasing the brake cylinder pressure to the atmosphere when the brake pipe pressure increases.  As an example the locomotive brake valve would charge the brake pipe to 70 psi.  The triple valve would charge the auxiliary reservoir to 70 psi.  Under these conditions the brake cylinder would be connected to a vent to atmosphere.  When the engineer wants to apply the air brakes he could reduce the brake pipe to 50 psi.  The triple valve would route auxiliary reservoir air to the brake cylinder until the auxiliary reservoir pressure gets down to 50 psi to match the brake pipe pressure.  In this particular case this would be a full service reduction and the final pressure in the brake cylinder would also be 50 psi.  This is known as equalization and is the maximum brake force available in an early brake system.  When the engineer wants to release the brakes, the brake pipe pressure is increased and the triple valve releases the brake cylinder air to the atmosphere and starts to recharging the auxiliary reservoir.  This recharging of the reservoir is done through a fairly small choke so air will continue toward the rear of the train and cause releases in other cars.  It is this slow recharging of the auxiliary reservoir that can cause a problem as another full brake application is not available until the auxiliary reservoir is fully recharged.   Modern railroad brake systems are far more complicated that this now, but this is the basic system that George Westinghouse developed in the late 1870s.  

An aside, the wreck of the PRR Federal Express into track 16 of Washington Union station, in 1953, occurred when the angel cock on the 3rd of 16 cars was shut, preventing brake line pressure reduction to 13 cars at the end of the train. So when the engineer applied the brakes, the rear cars shoved the entire train into the station and past the bumper post. 

Not that's what happened here and not a failure of the brakes per se, but an example of what did happen once.  In the case of the Federal Express, it was decided a flaw in design allowed the handle of angel cock to make contact with the car during motion and closing it.  

We don't know what happened yet, but the above is something that would be easy for investigators to check.

Bob