Shape of an X.
railroad crossing signs are railroad crossing signs, a baby could figure that one out.
A large X, referred to as a crossbuck.
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.
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.
Shaped like an X
Usually local ordinances.
It is red and white shaped like a cross.
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.
The purpose of a railroad crossing sign, is to let you know that there is a railroad coming up or that there is one right in front of you.
The Transcontinental Railroad
The sign stands for "Whistle Crossing". Many of these signs say "Begin WX" with the word begin on top of the WX. The sign also instructs the engineer where to start blowing his whistle. Engineer blows standard crossing sequence. Long, Long, short, Long.**ACTUALLY** to answer your question accurately, "W" signs stand for Whistle (for single Crossing). "WX" signs means Whistle for multiple crossings. Straight out of the NORAC railroad rulebook.