Guard rail (rail transport)

In rail transport, guard rails or check rails are rails used in the construction of the track, placed parallel to regular running rail to keep the wheels of rolling stock in alignment to prevent (or at least reduce the severity of) derailment. They are generally used along areas of restricted clearance, such as a bridge, trestle, tunnel, or level crossing. They also help to minimize damage to the structure and allow easier post-accident cleanup.

As per UK/Australian terminology, guard rails and check rails perform distinctly different functions. However, US terminology refers to both types as guard rails, despite their differing functions.

Check rails are located relatively close to the corresponding running rail (typically around 42 mm for standard gauge railways), such that the wheels of rail vehicles regularly contact the side of the check rail. Such is contact is essential for the function of the check rail, i.e. to prevent derailment of opposing wheels when passing through sharp curves or switches (turnouts). The distance from the contact face of the check rail to the opposing running rail is critical and referred to as the check gauge effectiveness (typically 1390-1392 mm for standard gauge railways .

On the other hand, guard rails are located further away from the adjacent running rail such that the wheels do not make contact with the guard rail under normal operation. The purpose of a guard rail is to guide a derailed wheel to maintain the derailed vehicle roughly in line with the track to prevent a more serious derailment. The spacing between the guard rail and adjacent running rail is usually somewhat greater than maximum with of the wheels, such that a derailed wheel can travel between the running rail and guard rail. Typical spacing is 200-380 mm for standard gauge railways.).