Because It's Friday

That is a 2-4-4-2 Mallet-type compound steam locomotive. Steam exhausted from the rear engines powered the front. That's why, on Mallets, the front set of cylinders are larger than the rear set.

Some say that the Big Boys are Mallets, in that they have two sets of engines. Big Boys are "simple", in that both the front and rear engines use steam directly from the boiler. My own opinion is that Mallets are compound locomotives.