The “Alleghenies” of the Chesapeake and Ohio Railroad were probably the largest steam locomotives ever constructed – or were they? They were certainly the heaviest, but the Union Pacific “Big Boy” was of an equal overall size. The wheel arrangement of the Alleghenies was 2-6-6-6, which included a very unusual 6-wheel trailing truck to support the enormous firebox. The size of the boiler made it impossible to position the pumps on the side of the locomotive, hence their presence on the front of the smokebox. What a monster!