running rail
One of the two rails of a railway track with which the wheels of a train make contact.
running rail: the steel rail that carries train wheels
The running rail is the load-bearing steel rail on which train wheels actually roll. On a standard two-rail track, both rails are running rails, distinguished from guard rails (which prevent lateral displacement) and conductor rails (which carry electrical current on electrified lines). The running rail supports the entire weight of the train and transmits it to the sleepers and ballast below.
Running rails are typically made from carbon steel and come in standard profiles. The rail head, the upper surface where the wheel flange and tread make contact, is hardened to resist wear. Modern heavy freight and passenger lines use continuously welded rail (CWR) laid in lengths of 100 metres or more, rather than jointed rail in 12-metre lengths, because joints create stress concentrations. The rail foot is anchored to wooden or concrete sleepers using clips, screws, or fastening systems designed to allow some lateral movement for thermal expansion.
Wheel contact pressures on the running rail head create rolling contact fatigue, which appears as small surface cracks that can grow into spalling defects. These defects are monitored by ultrasonic testing and visual inspection because they eventually cause rail breakage. The rate of wear depends on train speed, axle load, curve radius, and rail hardness. On sharp curves, the outer rail wears faster due to higher edge loading.
The profile of a running rail determines its compatibility with a wheel set. British Standard 11137, European EN 13674, and American AREMA standards specify different head shapes, foot widths, and weights per metre. A typical modern rail weighs 60 kg/m for mixed traffic, but heavy freight lines use 72 kg/m or heavier sections. Lighter industrial and heritage lines may use 45 kg/m or less.
Running rails are graded by their steel composition and hardness. Grade 900A is standard for most applications, but high-carbon grades such as Grade 1100 are used in switchwork and on severe grades where braking forces are high. Alloyed rails containing vanadium or niobium improve fatigue resistance but cost more.