[Venturer]

In trucks with air brakes, when the air pressure goes down the brakes come on automatically.
Evidently Trains are different.

Leaving a train with no one around seems a stupid thing to do. Any child or vandal could go on board and release the brakes. I cannot imagine something as expensive as a train left running with no one on board.

[Eric in the Ozarks]

Ditto heavy equipment.

[Roccus]

Missed your post while composing mine.

[Aqua225]

Trains are not different, their brakes go active as the brake line pressure falls.

However, it takes energy to activate those brakes. Each car’s braking system has a reserve tank that holds pressure to activate that car’s brakes. These do eventually leak down if not recharged from the locomotive. If line pressure had not recently been brought up (to unset the brakes at some point), these reserves would have failed here and there, causing the train to lose massive amounts of braking force.

Trains are often left alone. Railroads are private property and they have their own police. Generally, people don’t mess with trains because they don’t want to be arrested and prosecuted. Engines generally are left idling to keep their pure water coolant from freezing (pure water cools better than antifreeze mixture, but there is a freezing risk associated). Recent software for many engines now allow the ECM to cycle the engine on and off to keep the coolant from freezing, saving some fuel.

[Dalberg-Acton]

Same way on trains. Without air the brakes are applied. Not the way the article describes.

[freeandfreezing]

In trucks with air brakes, when the air pressure goes down the brakes come on automatically.

In air brake systems there are brakes which are spring loaded into the "stop" position, which are activated by loss of air pressure and are used as parking brakes, and brakes which are activated by pressure in order to move into the "stop" position.

It would seem that enough brakes would be of the spring loaded parking type to prevent motion of a train (or truck) with no air pressure present in the braking system, since this is a major safety feature of the systems. But apparently that wasn't the case in this situation, perhaps due to the grade where the train was parked or some other factor.

This accident is another example of how minor problems can compound into a disaster due to human error and ignorance.

[Mycroft Holmes]

In trucks with air brakes, when the air pressure goes down the brakes come on automatically.

Evidently Trains are different.

I went to a talk long ago and far away about train brakes. I also recalled the brakes being actuated when the pressure went away.

It's a little more complicated than that. The brakes are actuated on a reduction in pressure, but the motive force for the application of the brakes comes from an air reservoir aboard each individual rail-car, not from a spring.

A procedure called "fanning the brakes" can deplete all of the reservoirs and cause total brake failure. Don't do that.

[saintgermaine]

I always was under the impression that it takes “Air pressure” to unlock those brakes. Or on the other hand if the air pressure goes down the brakes will lock up. The only scenario I can imagine is if the locomotive is removed that there is a way for the brake lines to remain sealed and a certain amount of pressure remains and keeps the brakes unlocked until the air finally bleeds off and the brakes lock up. It be interesting to hear an explanation from an engineer familiar with this process.