rail head

The rail head is the upper surface of a railway rail, shaped to guide and support the flanged wheel of a train or tram. It is the only part of the rail that the wheel actually contacts during normal operation. The rail head carries the full load of the vehicle and generates the friction that allows wheels to accelerate, brake, and navigate curves without slipping sideways.

Rail head profile varies by application and region. Standard gauge heavy rail typically has a head width of 65 to 75 millimeters and a crowned or slightly rounded top surface. Light rail and tramway rails may be narrower, 40 to 50 millimeters across the head. The curvature of the head, its crown, is deliberate: it helps the wheel self-center on the track and sheds water away from the rail-wheel interface. Worn rail heads eventually lose this profile and must be re-profiled or replaced.

Wear, damage, and inspection

The rail head wears under constant mechanical stress. Vertical loading from the vehicle compresses the head; lateral forces from curves and wind load scuff it; impacts from wheel flats or rail surface irregularities pit and crack it. Shelling and spalling, where small chunks of the head flake away, are common failure modes in heavy-traffic lines. Corrugation, a rippled wear pattern that develops at regular intervals along the head, amplifies noise and vibration and accelerates damage. Track workers inspect rail heads visually and by ultrasonic testing to detect subsurface fatigue cracks before they propagate into the web or foot of the rail.

The rail head sits between the web (the vertical central section) and the foot (the wider base that anchors the rail to sleepers). These three parts form the characteristic I-beam or vignole profile standard since the mid-1800s. The head itself is harder than the web and foot because it must resist surface damage; rail steel is often heat-treated or composition-graded to achieve this hardness gradient without making the head brittle.

Lubrication of the rail head in tight curves reduces wear and noise and improves adhesion in wet weather, though modern traction control has reduced its use. On tram and light rail systems where the wheel flange rides on the outer gauge face of the rail head, the head is often lower and the gauge face more prominent than on mainline railway rail.