17) This picture, taken at one end of the marshalling yard at Whitemoor, in Cambridgeshire (the largest sorting sidings in Europe), shows the process in greater detail. The wagons come into the yard from this end. They’re then uncoupled and pushed over the brow of a hill known as 'the hump’ by a shunting engine, so that one wagon after another detaches itself from the rest and runs down-hill. All the sidings behind the control tower are numbered according to the part of the country they serve. In the tower a man works an automatic device which alters the points outside according to which siding, or road’ as it’s called, the wagons should run into. On each set of rails there is a retarder which is used to slow the wagons down to the right speed for running into the siding.