Siding

A siding is a length of track parallel to the main line, connected to it by a switch at each end, where a train can be held while another passes on the main. The basic operating purpose is to allow two trains heading in opposite directions to meet and pass on what is otherwise single track — a fundamental constraint on every line that isn't double-tracked. By extension, sidings serve as overtaking points, hold points for fleet management, and parking for failed equipment.

Sidings are sized to the longest expected train. On a busy single-track mainline carrying 12,000-foot freight, sidings of 14,000 to 15,000 feet are now standard so the entering train can clear both switches with margin. Older sidings of 5,000-6,000 feet are common on lighter lines and constitute a real operational bottleneck when modern long-train traffic tries to use them — the only solution is rebuild, often at considerable cost where geography is tight.

Single-ended sidings (or "stub sidings") are connected at one end only and used for industrial spurs, MOW equipment storage, and end-of-line terminals. Double-ended sidings are the universal passing variety. Where capacity needs to be high but cost low, sidings can be paired (two parallel sidings) or upgraded with intermediate signals so multiple trains can occupy them simultaneously.

In British operating practice the equivalent term is "passing loop" or, on busier lines, "goods loop" — the latter explicitly intended to let a slow freight clear out for a faster passenger train. The verb "to take the loop" is shorthand for being the train that pulls in and waits.

For railfans, sidings are some of the most photographable infrastructure on the network: they concentrate trackwork into a small area, they reliably stage meets and overtakes that you can predict from a scanner or app, and they often sit in scenic gaps in the timetable where two trains pause within a few hundred metres of each other.