When speaking of trains, humping refers to a method of sorting train cars. When you need to sort a set of cars onto different spur tracks, it would take quite a while to have a engine shove them in ones and twos to the individual sidings. So many switchyards have hills up which the cars can be pushed and sent rolling down the other slope one at a time. The switches are changed to have the cars coast onto the appropriate siding.
This is called humping.
You will sometimes see cars with No Humping signs on them. These are probably cars containing something breakable or alive (cattle cars, for example), which should not be crashing into other cars at the bottom of the hill.