Railroad Crossing (Crossbuck) Sign
MUTCD R15-1
Yield at the railroad crossing; stop if a train is approaching.
What it looks like
A white X-shaped crossbuck with the words RAILROAD CROSSING in black lettering across the crossed arms, mounted at the track.
When you'll see it
You see it standing directly at the point where the road crosses railroad tracks, often alongside flashing lights or gates at busier crossings.
What to do
You must yield at the crossing and be ready to stop; come to a full stop if a train is approaching and never cross until the tracks are clear.
Related signs
Source: Manual on Uniform Traffic Control Devices (MUTCD), 11th Edition — FHWA.