It refers to a common method used to sort freight cars known as "humping," which involves the use of a man-made hill, or hump. A track heads up the hill and branches into numerous parallel tracks on its way down the other side. To make up new trains, a switch engine pushes a string of cars to the top of the hump, where the cars are uncoupled one at a time. Having determined the car's destination, a worker in a nearby tower pushes buttons or throws levers or whatever to get the track switches (you know, those things where one track divides into two) lined up properly. The car is then given a nudge, causing it to roll down the hump and onto the right track.
There's gotta be a song in this somewhere...
