This flatcar, designed to transport highway trailers, is labeled "DO NOT HUMP" to protect the car and its load from the jolts common to a freight-car-sorting process called “humping.” During humping, a string of freight cars is pushed over a man-made hill called a “hump” and one-by-one the freight cars are allowed to roll down onto location-specific tracks on the far side of the hump. On those tracks they coast until they couple onto cars already sitting there or until the gradient stops them.

The instruction tells railroad employees in a hump yard (a freight yard having a hump) not to allow this car to be pushed over the hump. Certain loads are too delicate or too volatile to be subjected to the impacts of car-coupling in hump yards. The pictured cars are designed to carry highway trailers. They are not to be humped for one or more of these reasons: the cars themselves are too delicate or the highway trailers are too delicate or the hardware for connecting the trailers to such flatcars is too delicate.

Some freight cars, because of the loads they always carry, are permanently labeled with the instruction. The pictured flatcar falls into this category. Other freight cars sometimes transport “delicate” loads and have "DO NOT HUMP" signs temporarily affixed to their sides.

This scene is just south of BNSF’s South Seattle Yard, a trailer transloading facility. The view is to the northwest and the Interstate 5 bridge over the tracks is visible in the break in the trees.

An aerial view of a hump yard helps to understand the notion of location-specific tracks one side of a hump. Conway Yard, in Conway, Pennsylvania, is a huge facility owned by Norfolk Southern Railroad. Its hump is at the center of this Wikimapia aerial view. Freight cars to be humped are pushed from the south over the hump (note the pedestrian tunnel under it) and directed onto the appropriate track on the north side. Each track could represent a particular major destination.