The railroad crossing warning sign called a crossbuck is shaped like this letter
Crossbucks usually are a traffic sign to indicate level railway crossings, 
sometimes supplemented by electrical warnings of ... signs of a style similar to 
those used in the U.S., except the word "RAILWAY"... [DOC] [TLE] Driver Education Signs - National Driver Training [PAR] Usually diamond-shaped and yellow, warning signs caution drivers that the road 
is slippery when ... A crossbuck indicates the railroad crossing.