A:  This is a very good question, and it is one of the things that we study a lot at Birket Engineering when we design control systems for roller coasters.

If it is a modern roller coaster, the brakes are designed so that no matter what goes wrong with them, they CLOSE. In other words, it takes a strong signal and lots of air pressure to force them to open when a train is coming, to let the train pass the brakes. A lot of thought goes into the design of the brake, the air valves, the wiring and the computers to make sure that if ANYTHING goes wrong, the brakes CLOSE, so that the train stops at the brake. We don't take any chances on this.

We also make the logic (rules in computer software and electrical wiring) so that the brake only gets the command to open if EVERYTHING is working correctly. Even then, we use a lot more brakes than it takes to stop the trains, just to be sure. On rides with high risk, we make extra rules for ourselves to follow in the logic of the ride, like "always keep at least two groups of closed brakes between loaded trains."

This question of yours is one of the most serious areas of study for a roller coaster control engineer. Hundreds of hours are devoted to this question in the design of each new coaster, if the coaster is very complicated. Several engineers work together on the problems, and check each others work often.