A warning to always take care with what you read at Wikipedia.

http://en.wikipedia.org/wiki/D1_class_Melbourne_tram

"All D class trams have a traction braking controller with an integral deadman's trigger that the driver must release and reapply every minute. If an alarm sounds they must release and reapply the trigger immediately; otherwise, magnetic track brakes are applied at full force."

Not so. The disc brake applies firmly but not harshly and only after an alarm signal sounds four times, over about four seconds.

Andrew C.