running line
A railway line used to operate trains on, as opposed to a siding.
running line: the main track where trains move
A running line is the primary track on a railway where trains operate under normal service conditions. It carries scheduled passenger or freight traffic and is maintained to the standards required for regular operation. The term distinguishes it from sidings, spurs, or other secondary tracks that branch off for specific purposes like loading, storage, or passing.
Running lines are typically laid to tighter tolerances than sidings. Rail gauge must be held within strict limits, usually plus or minus 6 mm on standard gauge (1,435 mm). Curves have defined radii and superelevation angles to permit safe speeds. Ballast depth and compaction support heavy repeated loads from passing trains. The track bed itself must drain properly to prevent frost heave and differential settlement that would cause rough riding or derailments at operational speeds.
Speed, signalling and control
Each running line has an assigned maximum speed, which may vary by section depending on alignment, gradient, and infrastructure. This speed is enforced through track geometry and signalling systems. Running lines are equipped with signals, block sections, and in modern systems, automatic train protection or cab signalling. Sidings and spurs typically operate under lower speeds and may use simple hand signals or no signals at all.
A railway may have multiple running lines running parallel to each other. On busy routes, separate lines may handle opposite directions of traffic, or additional lines provide capacity for express and local services. Each line has its own drainage, ballast, and signalling. The cost of building and maintaining running line infrastructure is substantial; many railways have rationalized routes by removing parallel lines to reduce maintenance burden.
The term running line appears in operational planning, scheduling, and engineering. Planners refer to line capacity, meaning how many trains per hour a running line can safely handle. Maintenance crews know that running lines require more frequent inspection and renewal than sidings because they bear concentrated traffic loads. When a section of running line fails, service across that route stops entirely.