check rail

A check rail is a short length of rail installed parallel to and slightly inset from the main running rail, positioned to engage the outer flange of a wheelset and prevent it from climbing the gauge face of the running rail. It is used at locations where wheels experience lateral forces strong enough to risk flange climb derailment: at points (switches), crossings, and on sharp curves. The check rail typically runs 1.5 to 3 metres and sits 6 to 16 millimetres from the running rail, depending on track gauge and design speed.

On a points assembly, the check rail guides the wheelset as it enters the switch, holding it firmly against the stock rail (the undeviated rail) or the switch blade (the movable rail), so the wheel does not climb outward during the transition. At a crossing where two running rails intersect at an angle, check rails flank both approaches to stabilize the wheelset as it passes over the nose of the crossing frog. On curves with radius below 500 metres, check rails on the outer rail prevent the centrifugal effect and wheel binding from forcing a flange up and out of gauge.

The check rail is secured to the sleepers (ties) using the same fastening method as the running rail, though it often uses a lower rail section and lighter clips since it carries no vertical load during normal running. In older track, check rails were bolted as a separate item; modern track increasingly uses a raised check rail cast integral with or welded to the switch blade itself, eliminating a separate component. On sharp crossings and tight points on busy lines, a double check rail (one on each side of the running rail) may be installed to control lateral play in both directions.

Failure and wear patterns

A check rail wears fastest where it regularly contacts wheelset flanges; the contact line becomes shiny and polished. If the gap between check rail and running rail is set too wide, the wheelset can rock sideways and strike the check rail with shock loads, causing bending or cracking at the foot of the check rail near its mounting points. Corrosion and loosening of fasteners also allow the check rail to drift out of position, reducing its guiding effect and increasing derailment risk. In frost-prone areas, water trapped in the gap can freeze and expand, lifting the check rail or causing the fasteners to work loose.

The term 'check rail' derives from its function to check, or arrest, unwanted lateral motion of the wheel. In North American practice it is sometimes called a guard rail, though this term also refers to protective rails beside the track. The check rail is one of the oldest passive safety devices in rail infrastructure, fundamental to the reliability of modern mixed-traffic rail networks where speed and traffic density demand positive wheel guidance.