A serious accident resulting in fire and injuries to Fire Fighters in engine-73's station. Members were preparing a reserve piece for service when it jumped into gear and ran into the wall between the bay doors, injuring the eng. Lt. and 2 other members. All hands were in svc using several hand lines and a deck gun. Rescue-1 and Squad-72 were in svc for extrication of the members and shoring up the bldg. The bldg sustained heavy fire and structural damage. 2 of the Fire Fighters sustained serious but non-life threatening injuries.