Re: Drivetrain Question
 

If you are going wth pneumatics or brakes, just use spring loaded cyclinder or mechanism that deploys the brakes in an unenergized state. Use a single solenoid, to retract/release the brake when under electric control. loss of power, loss of pneumatics will deploy brakes.

Look up rail cars, trailer brakes, they use air pressure to release the brakes. Lose the air pressure and the rail car stops, not continuing down the tracks...