I recently discovered that my companies shunting tugs, used for moving 36 tonne road trailers, have a park brake that applies automatically when the ignition is turned off or fails unexpectedly. It also releases automatically when the ignition is switched on again if the park brake lever is not manually applied!
The tractors drive axle park brakes will fully apply even when the truck/trailer is at full speed and otherwise unbraked.
Is this safe given that the road/load conditions are entirely random?

The problem came to light when the ignition system failed and, due to a faulty isolator switch, the handbrake could not be applied at all.