Hydraulics On a Roller Coaster

Generally there are no hydraulics, only pneumatics. There are multiple brakes throughout the course which are kept in the closed position by springs. Hence the brakes are always "on". Air pressure from the pneumatic system is used to force the brakes open so that the train can pass thru that segment or "block" of track. The pneumatics operate much the same as on a semi trailer truck.

A fallacy perpetrated in the movies is that oil-based hydraulics force the brakes closed and when that fails the train moves thru the course freely. Just the opposite is the case... at least, as far as I know. If there are any cases of a roller coaster which actually uses oil instead of air, or use a control system that forces the brakes closed rather than open, then someone please correct me here.

I think an online search will probably be your best source for info, now that you have a better picture of how the safety systems actually operate.