advtrains_supplemental/advtrains_train_rocket: Stephenson's Rocket and barrel wagons for AdvTrains. Originally at https://github.com/mbruchert/advtrains_rocket

mbblp 6340b70239 add barrels		7 lat temu
..
advtrains_barrel.b3d	78dd5abf32 add barrels	7 lat temu
advtrains_barrel_stack.b3d	6340b70239 add barrels	7 lat temu
advtrains_rocket.b3d	e3f21734fb rocket	7 lat temu
advtrains_rocket_wagon.b3d	e3f21734fb rocket	7 lat temu
advtrains_rocket_wagon_long.b3d	e3f21734fb rocket	7 lat temu