What shape is a standard US railroad crossing sign?

A standard US railroad crossing sign is a crossbuck.

Its two white boards form an X and traditionally carry the words “RAILROAD” and “CROSSING.” The sign marks the point where a road crosses railway tracks at grade. In the United States, public crossings are required to have at least a crossbuck, while busier locations may add flashing red lights, bells, and gates.

The crossbuck is not the same as the circular yellow advance-warning sign placed farther from the tracks. The yellow circle warns that a railroad crossing is ahead; the crossbuck identifies the crossing itself. A supplementary plate may state how many tracks cross the road, especially where gates are absent.

The X-shaped design is used internationally, although colors, wording, and legal classifications vary. In Canada, for example, crossbucks commonly have a red border and no words, while US versions usually use black lettering on white reflective material.