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Whistle posts warn locomotive engineers where to begin sounding the horn for grade crossings.

A whistle post (or whistle board), in railroad usage, is a sign marking a location where a train's engineer is required to sound the horn or whistle.

Whistle posts in the United States were traditionally placed about 1,320 feet (one-quarter mile) in advance of a road crossing.

Whistle post as used by the Southern Railway.

The signs in themselves varied in design from railroad to railroad. Some were marked with - - o - (two longs, one short, and another long) in the manner of Morse Code. This sequence is known as Rule 14L in almost all railroad operating rules.

Modern whistle posts are of simple sheet metal construction, utilizing a 'W' marker. Multiple crossings protected by a single sign have an 'X' displayed below the 'W'.

Whistle post which protects multiple grade crossings.

Whistle posts used on the former Southern Pacific display an 'X'. Multiple crossings have a number displayed beneath the 'X' for the number of crossings.

Modern whistle boards in the UK comprise a white circular sign bearing a letter 'W'. Early whistle boards generally had the word "Whistle" written in full on a rectangular board. The Great Western Railway's signs had 'SW' for "Sound Whistle".

One modern variation found in Scotland is the 'continuous' whistle board, comprising a white rectangle with the 'W' below a diagonal cross. The driver must sound the horn continuously on the approach to the level crossing ahead.

In Ireland, a whistle post is an upright rectangle bearing black and yellow diagonal stripes.

In France, a whistle board comprises a black rectangular board bearing a white letter 'S' for "Sifflez" (= "whistle"). An additional white board with a black 'J' for "Jour" (= "day") indicates that the sign does not apply at night time.

In Germany, a rectangular board bearing the letter 'P' for "Pfeifen" (= "whistle") is used as a whistle board. It may have either a black 'P' on a white background, or a white 'P' on a black background. Two boards, one above the other, means "whistle twice".

Where an additional sign with two vertical stripes is mounted above the 'P' sign, that sign only applies to trains that are not stopping ahead (e.g. at a station).

A whistle board on the approach to a level crossing at Somonino, Poland.

In Poland, a whistle board comprises a white triangle with a black border. A whistle board associated with a level crossing depicts the silhouette of a car in the middle of the triangle.

In the People's Republic of China, a whistle board is a white diamond with a black border with the character míng 鸣 (whistle) on it