All our large diesels had an air cut off valve. None of them would run without air.
The shut off solenoid in my 12V stuck a couple of years ago... It was definitely an interesting experience, but fortunately not truely a runaway situation:
Turn the key off; engine still running. 
Remove key; engine still running.
Put it in 4th gear and popped the clutch; engine stopped. (whew) 
I had a 79 300D for a short time that didn't always turn off with the key. Mercedes cleverly placed a lever in the engine compartment that would cut the air supply and kill the motor. Even if that wasn't there, I don't think the 88 hp that car had (when new) would've been too hard to deal with...
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