n. On a railway-track, an additional rail placed beside the rail in service, either with the object of receiving the wheel in case it should leave the track or of preventing the wheel from leaving the track.n. Nautical, a fore-and-aft timber bolted on the outside of the covering-board or plank-sheer on steam-vessels navigating harbors, lakes, and rivers, to act as a fender when lying alongside of other vessels, or when made fast to a dock. Sometimes a second guardrail is carried along the sides just above the water, and is then called a bilge guard-rail.