A hump, or hump yard, uses gravity to help sort cars. Like in any classification rail yard, trains from a common origin are broken down and sorted into new trains for common destinations. In a conventional or "flat switching" yard, this is done by switch crews moving back and forth on a switch ladder. Cars for destination "A" are put on track 1, "B" on 2, etc. This requires a lot of forward and reverse moves.
In a hump yard, cars to be sorted are pushed only forward, up an incline. The top of the incline is located at the beginning of the ladder. As cars reach the summit, they are uncoupled. (All of the air brakes must be bled off before this takes place.) Switches are aligned to route the car into the proper track as it rolls down the other side.
In some yards, devices called retarders slow the descent of the cars as they enter the classification tracks. This ensures even speeds and safe stops at the other end or when coupling into other cars on the track. In earlier days, men would ride the cars into the bowl and stop them with handbrakes. Since the 1960s, computers have been used to control retarders, track cars and monitor capacities.
The other end of the classification, or sorting, tracks climb back up a short grade. This keeps cars from rolling out the other side. Consequently, this part of the yard is often called the "bowl."
Switch crews pull cars out of the other end of the bowl and assemble new trains which are normally placed in a departure yard to receive locomotives, cabooses (in the good old days) and crews. Outbound trains may be blocked for multiple destinations to speed work along the way.