(From the article) Trains need sand, water, coolant, and other materials for normal operation. These need to be regularly resupplied (at depots). Hence, a typic

(From the article) Trains need sand, water, coolant, and other materials for normal operation. These need to be regularly resupplied (at depots). Hence, a typical system would be to dedicate each locomotive engine to a stretch of track (the length of which was constrained by how long the train could go without a resupply) between two depots. The train would stop at the depot, move the cars to another engine, send the new engine along, resupply the old engine, and have the old engine take new cars from yet another train along the track in the opposite direction (to the depot it originally came from).