hump

In rail yards, the hump is a gentle artificial hill, typically 40 to 60 feet high, built into the track layout to separate incoming freight cars by gravity. A locomotive pushes a cut of cars up and over this rise, then releases them. Once they crest and begin rolling down the far side, gravity takes over, and the cars coast into different sidings based on which switches are lined for their destination. This process is called humping, and it is how large classification yards sort hundreds of cars per day without the fuel cost of individual engine moves.

The hump yard itself is the infrastructure: the summit track, the lead track where cars are assembled before being pushed, the retarder systems (air or hydraulic brakes) installed on the downhill tracks to slow cars safely, and the ladder track on each side where cars are sorted into 30 to 60 different classification tracks. A typical humping operation moves one car every 12 to 20 seconds during peak activity. The operator in the hump office controls the switches and retarders from a control stand, sometimes with a brakeman or switch attendant positioned in the yard to observe movements.

Humping works best with cars weighing 60 to 100 tons; very light or very heavy cars behave unpredictably on the slope and require manual switching instead. Certain cars, such as those carrying hazardous materials or refrigerated goods, may be banned from humping to avoid damage or spillage. Wind speed and rail temperature also affect rolling distance, so operators adjust retarder settings seasonally and monitor weather.

Why it is called the hump

The term is literal: early American rail yards in the 19th and 20th centuries built physical earth or gravel mounds in the middle of classification yards. As yards grew, these were formalized into permanent structures with steel rails, concrete, and then automated control systems. The word hump persists even though modern yards are more engineered than the original bumps.

Major freight railroads operate fewer hump yards today than decades past, partly because intermodal containers and unit trains (dedicated trains of identical cars) reduce the need for frequent classification. However, traditional class yards with humps remain essential for handling mixed freight, and the term humping is still used across North American rail operations. The skill of reading car behavior on the slope, judging when to cut them loose, and timing switch throws remains a core discipline for yard workers and operators.