The Slip Coaches are Duck's old coaches from the Great Western Railway.
A slip coach or slip carriage is a British and Irish railway term for passenger rolling stock that is uncoupled from an express train while the train is in motion, then slowed by a guard in the coach using the brakes, bringing it to a stop at the next station. The coach was thus said to be slipped from its train. This allowed passengers to alight at an intermediate station without the main train having to stop, thus improving the journey time of the main train. In an era when the railway companies were highly competitive, they strove to keep journey times as short as possible, avoiding intermediate stops wherever possible.