What is "crossbuck"

n. A sign consisting of two slats on a pole forming a diagonal cross, usually indicating a level crossing.

A crossbuck is a sign composed of two slats of wood or metal of equal length, fastened together on a pole in a saltire formation (resembling the letter X). Crossbucks usually are a traffic sign to indicate level railway crossings, sometimes supplemented by electrical warnings of flashing lights, a bell, or a gate that descends to block the road and prevent traffic from crossing the tracks.