Ah, the inevitable railroad jokes. Humping, in railroad terminology, means (if I recall correctly) pushing cars onto a hill where they are uncoupled one by one, then they roll into the proper sidings directed by a switchman. It’s faster than “manually” sorting them, but the disadvantage is that it’s tougher on the cars and their loads. Some fragile loads, or cars that might be heavy enough to cause damage with their impact, are labelled “do not hump” for this reason.