I was reading about the Lac-Mégantic rail disaster and was a bit confused about the lack of air pressure to the train’s braking system causing it to runaway. I’d always assumed that the air pressure held the brakes open against a strong spring that would normally apply the brakes. I realize that’s not the case (though it’s pretty much the way they work on big rigs and other road vehicles with air brakes). Instead there’s an air reservoir in each car that stores pressure to apply the brakes, and that air eventually bleeds away if it’s not replenished.
Obviously the problem at Lac-Mégantic is that with the locomotive turned off because of a fire, there was no air compressor to keep all the brake lines and the individual car reservoirs topped off. It only took an hour from that time before the pressure had dropped enough for the train to start rolling. That said, some of the tanker cars that didn’t derail were pulled away from the fire the next day, and their air brakes were still engaged, so they had to either use a special switcher locomotive (I assume with its own air compressor) or manually dump the air out of the reservoirs to move those cars.
So I’m wondering, what’s a typical expected time for the air brakes in a train to remain useful after an emergency application? Say you have a mile-long train on an uphill grade with just a two-man crew, and some cars at the back come uncoupled. Do they hustle back there and apply the hand brakes as soon as possible, and maybe chock the wheels too while waiting to get the new coupler and hoses installed? Obviously a poorly maintained train can bleed air out pretty fast, but individual cars can theoretically hold their pressure for hours if not days. What’s typical railroad policy for such situations?
Also, in reading up on it I wondered why springs aren’t used on railroad brakes. It sounds like there’s some situations where they are, especially in passenger trains, but do they even have the strength to work on a freight train? If not, I would think they could at least be used as a parking brake, which I believe is technically what they are on big rigs. Still, that would make yard switching much more difficult, and hump yards would probably be completely unworkable without some way to disable the brakes, and with a spring strong enough to stop a freight car I assume that’s a non-trivial problem to solve.
Air is quite springy. And springs get stiffer, the further down you push. If you had to work the air against a spring, you don’t get the sharp hard braking that “air brakes” are known for… Also, you can’t just run that system from the engine, because it takes so long for enough air to get down to the last car that the end cars run into the front cars.
Instead, Westinghouse brakes were presurised on both sides, and you release the air on one side to apply the brakes. There was an air cylander on every car, and that was kept pressurised by an air line from the engine – which required coupling and uncoupling when you couple and uncouple the car, which is why automatic coupling and uncoupling is so difficult.
(His first system had a single vent at the engine – it controlled all the brakes. Then, in order to make it work even better, Westinghouse developed the system where, under pneumatic control, each brake is vented at the car,: air to opperate the brakes doesn’t have to get down to the last car in sofficient volume and pressure, or up to the engine in sufficent volume: you just need enough control air to opperate the valve at each car)
The resevoir and brakes on each car shouldn’t leak out in an hour ??? Maybe that was because of the accident ??? But a train should be parked on the level, and the parking brakes (dunno what they are called) should be applied.
It can be a bit complicated, as train air brakes don’t work quite like truck brakes. As I understand it, from articles in Trains and Railfan magazines, the brake issue is related to setting the handbrakes. For a given yard, there will be a % of cars mandated for parking a train without connection to an air source. Railroad cars have what is effectively a parking brake. It’s usually called a handbrake, and is activated by a big wheel (the brakewheel!) on one end of a car, which winds up a chain and mechanically connects to the brake rigging under the car. When a car is tied down (as it’s called), the brake shoes are forced against the wheels by the chain and linkage. The state of the air on the car becomes irrelevant. IF THE CAR IS PROPERLY SET, adding air means nothing, to move the car will require releasing the handbrake. What may have happened is that the crew did not set the brakes on enough cars (say 10 instead of the required 20). On the grade involved, that was enough that when the air bled off the other cars, the whole affair started moving downhill, with the few cars with brakes on dragging with locked wheels. When the switcher went in to get cars with the brakes set, it had to simply drag them out with locked wheels (cleans the rails, though). Or manually release the handbrakes. It does not take a special engine to release the train brakes, any engine can do that, they all have compressors.
I think it would more likely to be a control and booster situation… like on a truck… one is the control air hose and the other is a supply air hose,
The control air hose has very little flow, so as to ensure that the set level of braking is accurately communicated down the train smoothly.
The train didn’t have a safeguard on the park brake, so the engineer had to go from wagon to wagon to apply them. He didn’t want to cause extra hassles by applying the park brakes on all wagons, as some of them may jam. (due to lack of use and maintenance, a few would probably get jammed on and so have to be dismantled to move the train …) … So he applied a minimal number, he used what is fine for a section of wagons on a flat, he didn’t use enough for the entire train on a bit of a slope.
It seems the engineer didn’t understand the air brakes, it sees he thought that the air brakes failed as soon as the compressors were off!. So he noticed that the train didn’t roll away, and so he assumed the minimal, seven, manual emergency/wagon stabling brakes were enough.
I don’t know anything about the operation on air brakes on a train, but I would like to explain some things about the Lake Megantic disaster. It was a combination of factors:
The only crewman, the engineer had been working for something like 15 hours.
To set a hand brake required he climb up a ladder on the side of a car and crank a handwheel. Climb down, repeat.
He should have set hand brakes on about 20 cars; he did only 10.
He left the engine running to keep the brakes pressurized, and went to bed.
The engine caught fire and the firemen (or maybe the local dispatcher) turned the engine off after the fire was put out.
The engineer was actually awakened by this, but the dispatcher told him he wasn’t needed and should go back to sleep.
When I was a freight train conductor (and I believe now still) there’s a rule/procedure governing how many Hand brakes should be applied to hold the cars standing whether there’s an engine attached or not.
It’s something like 10% or a sufficient amount to hold the train. That means applying what you think is enough handbrakes and then releasing all other brakes, wait a certain amount of time to visually see whether or not the train starts to roll away. Repeat.
He probably applied what he thought was enough. The engine shuts down. Brakes bleed off. Train rolls away.
It comes down to hours of service laws and crew issues I suppose.
Back to my original question though, how long is a reasonable expectation for air brakes to hold assuming a failure or loss of connectivity? I guess that would be more of a procedural/policy question than a technical one. How long can individual cars hold their pressure? At Lac-Mégantic enough of the cars lost their air brakes that they couldn’t hold the train in place anymore, but several were still engaged the next day, and from what I hear some cars on customer sidings can still have their air brakes engaged days after delivery.