In North America, they're round.
A large X, referred to as a crossbuck.
railroad crossing signs are railroad crossing signs, a baby could figure that one out.
Railroad crossing signs are either a white x shape with the words railroad crossing inside of them or a yellow circle with a black x and the letter R on the left and right side.
Some have special shapes, such as the octagon for the stop sign and the X shape for railroad crossings. Some signs can be localized, such as no parking and some are only found in state .
An X. Because it is a cross for the railroad crossing.
The road sign is a large yellow circle with a black X across the circle. The railroad can also erect their own RR crossing warning signs which consist of a two planks of wood (or other material) configured into a shallow-X or "crossbuck" style sign. These warning signs are painted white with the words "rail road crossing" stenciled on them in black letters.
Most mandatory road signs are circular
WHAT icons? If you are referring to the SHAPE of the signs themselves, the shape of the sign itself signifies the type of information the sign conveys. (e.g.: Octagaonal signs are ALWAYS stop signs - Diamond-shape signs ALWAYS convey some type of warning message - etc).
a triangle
Usually local ordinances.
An advance railroad sign is round and has a yellow background with a black X and black R's on each side of the X. The sign at the site of the tracks is an X shaped sign that is white with black text and says "Railroad Crossing" with the words crossed on each leg of the X. Crossbuck.See the related link for a site with pictures of US traffic signs.