Chopper control DOES also relate to the braking system, seeing as Melbournes tie stations can't support reflux feeding, the secondary system on the train comes into play, which is all aspects - is a rheostatic brake.
Chopper control is basically a magnetic phase shifter, it uses a quick series of 'on' 'off' to smooth out the current going into the motors. They are quite prone to faults on a system that isn't designed for them in mind. From memory 692M has spent more time off the road than on the road in its whole life to date.
A train has 4 different brakes - Rheo, E.P. and Auto Air and a park brake, all serve a different functions vita to stopping the train, and acting as a fail-safe device.
Rheo, any speed from 115km/h to anywhere between 15km/h to 5km/h will be used to slow the train down. Involves using the speed of the train, to create a magnetic field oposite to the phase of the traction motors. Causeing a repelling force and hence slowing the train down.
E.P., or Electro - Pneumatic. Is used as the secondary brake, or primary brake if the driver chooses so. Involves the brake handle switching relays, which then lets a certain amount of pressure into the brake cylinders causing the brakes to retard the forward motion of the train.
Auto Air, is the last of the brake systems used to stop the train from a moving speed. Not like the other systems, this system does NOT require power to operate. You technically have 3 'bites of the cherry' befre you run out of air (if there is no power to run the compressor). If all other systems fail, this one takes over. Involves the balancing or air pressures via the brake handle, aux resivoir and the triple valve.
Park brake is just to hold the train still. A spring will hold the brake in the 'on' possition until there is enough air pressure for the spring to be held back by air pressure in the cylinder, and therefor enough pressure to operate the brakes normally.
Hope this helps.