Had a locomotive do that, scary part was, everything was off, shouldn't have been possible for it to start, but it sure did! I've had railroad cars just start rolling away on me too, even ridden one runaway group of MOW equipment!
My mom was backing a pickup and a boat trailer into the lake, with my dad trying to direct from in the boat on the water, finally she gets mad and jumps out of the truck, only to have it start rolling into the lake! she just managed to hit the parking brake as the rear of the cab enters the water!
Always chock and check your parking breaks!