BOTTOM DOOR ON A LOADED RAPID DISCHARGE AIR DUMP RAILCAR WAS STUCK IN CLOSED POSITION. THE EE INSPECTING THE CONDITION OF THE DISCHARGE PISTON PLACED HIS RIGHT-HAND ON THE END OF THE PISTON SH AFT. THE DOOR INADVERTENTLY OPENED WITH SUFFICIENT FORCE BREAKING ONE BONE OF THE RIGHT-HAND BELOW THE LITTLE FINGER NEAR THE WRIST.