Speaking ONLY for MY early years here there was a difference. The Engine carried SOME hose,booster tank and the nozzless,Deck gun and the 2 Scotts.The WAGON carried hose,1800' of it, 250 tank,almost NO appliances,and two ground ladders. As the others have alluded it was a HOSE wagon,designed to connect a hydrant to the attack engine or connect Two Engines. GENERALLY didn't do anything else so it wasn't considered an Engine. T.C.