lye

A lye is a short section of track that branches off from the main line, allowing trains to move onto it without blocking through traffic. It serves as a passing siding, storage track, or connection to industrial facilities, yards, or other rail infrastructure. The lye is controlled by a switch (or points) that diverts a train from the main line onto the branch and back again.

Lyes vary in length and purpose. A short lye might be 200 to 400 feet long and serve only to let one train stand clear while another passes on the main. A longer industrial lye may extend 1000 feet or more to a loading dock, mine portal, or warehouse. The gradient of a lye is usually gentler than the main line to prevent loaded cars from rolling, and the track geometry must accommodate the operating speed of trains using it without excessive wear on the outer rail.

A dead-end lye (or stub siding) connects at only one point and is used for storage or loading; a through lye connects at both ends and allows a train to enter from one direction and exit toward the main line going the other way. The switch that controls entry to a lye may be manually operated by a signaller, remotely controlled from a cabin, or (on busier lines) electrically powered. The track inside a lye must be maintained to the same standard as the main line to prevent derailments, though the ballast and sleepers often deteriorate faster due to less frequent inspection.

The term lye is chiefly British and Irish railway vocabulary; North American railroads more often call this a siding. The word itself has uncertain etymology but appears in railway documents from the 19th century. A lye differs from a loop line in that a loop allows two trains to pass each other simultaneously on a single-track main line, whereas a lye is simply a branch where one train waits.

Operational hazards on a lye include hand-operated switches left in the wrong position, fouling of the main line by cars left too close to the switch points, and brake failure on graded lyes. Lyes are also common locations for unauthorised access to trains and theft of cargo, so visibility and fencing matter to safety.