Hi Caveman,
No, a Loco shouldn't be able to take a high velocity 75mm AP round and keep going. The Boilers where easily penetrated by .50 cal AP and the explosions when hit are quite spectacular.
If you watch gun camera films of P-51s and 47s strafing German locos you'll see the boilers being hit and great clouds of steam shooting up the smoke stacks and out the sides. The box cars also probably would go up with 1 HE hit, but heaven help you if you hit an ammo car from within 500 yards. In that case you would probably have created the first truly airborne Panzer.
- Andy