A rail (fence) erected to restrain vehicles which are out of control.
A longitudinal rail running alongside a railway track and raised in height above the running rails. Guard rails are usually found on bridges and on sharp curves and are intended to restrain the lateral movement of vehicles which might become derailed.
A Guard rail is a system designed to keep people or vehicles from (in most cases unintentionally) straying into dangerous or off-limits areas. A handrail is less restrictive than a guard rail and provides both support and the protective limitation of a boundary.