Peak service on London Underground is done by "stepping back". You bring a train in, switch off (allowing your driving cab to become a trailing cab in a second) and step out of the cab, then another driver steps in at the far end, switches that on, and the train announces its new destination and reconfigures appropriately. You begin walking down the platform. The train leaves, but in the time it takes to stroll to the far end another one arrives, so you get in and drive that one away. Thus drivers "step back" by one train each time they do this.