[thirteenfingers]

"The air hoses between the locomotive and the cars were not connected, as is normal during this kind of switching operation. The air brakes on the cars were therefore inoperative."

This part confuses me. I was under the impression that modern train air brakes are fail-safe, i.e. pressure will release the brakes, no pressure will result in them being applied, so that the cars couldn't be moved in the first place without the air hoses between locomotive and cars being connected. Can someone with a better understanding of rail operation help me out here?

[p_l]

From my understanding, most modern vacuum brakes involve two pressure loops with two cylinders on each brake. When connected and operating, one cylinder is responsible for brake force and the other acts as counter to it - in case of disconnect, the countering cylinder evacuates fast and results in braking.

However, if the system is not "primed" (for example after longer stay disconnected which gets both cylinders empty) the brakes are inoperational except for manual mechanical brake one is supposed to apply for any longer stay.

[fredoralive]

The air brake system is fail safe against accidental disconnection. You can intentionally get around it if needed. Obviously they did something like that here, expecting it to be a safe setup for low speed shunting in a yard...

[msisk6]

All US rail freight cars have a "bleed rod" that releases all the air in the brake system so you can move cars for switching.

Most large railroad yards nowadays are "hump yards" where a locomotive pushes cars over a small hill and they roll down the though a series of switches with a computer monitoring their progress and activating rail mounted brakes that squeeze the sides of the wheels so they roll into other cars at a controlled speed to make up trains.

Smaller yards, like in this story of the runaway, are "flat switched" where a locomotive and man power manually do all this work to put a train together. Back before railroads started reducing train crews the engineer wouldn't need to get out to align switches; they had a "switchman" who would get out and align the switches as needed.

[jvolkman]

And if you ever see "DO NOT HUMP" on the side of a car, this is what it refers to.

Crews also "kick" cars in flat yards which looks a bit more like humping in that brakeless cars roll down tracks on their own: https://youtu.be/zEWE3df6Q2s?t=248

[joncrane]

That is an amazing video. Thank you.

[flyinghamster]

Also, fail-safe only works so far. A parked car needs to have handbrakes set or wheels chocked, since the air can eventually bleed out of the reservoir and release the brakes. The Lac-Mégantic disaster [0] involved insufficient handbrakes set on a train parked on a grade.

[0] https://en.wikipedia.org/wiki/Lac-M%C3%A9gantic_rail_disaste...

[michaelt]

I too was under the impression trains could not move without the brakes connected.

https://en.wikipedia.org/wiki/Railway_air_brake describes a "fail-safe" system which has been "nearly universally adopted" which might give you some details.

The diagrams are very complicated - you'd think it would just be a spring-applied brake with air disengaging it, but it seems to be a much more complex system.