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A whistle post (or whistle board), in railroad usage, is a sign marking a location where a locomotive engineer is required to sound the horn or whistle.
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United States and Canada
Whistle posts in the United States and Canada were traditionally placed about one-quarter mile in advance of a road crossing.
The signs in themselves varied in design from railroad to railroad. Some were marked with – – o – (two longs, one short, and another long) in a similar manner of sending the letter Q in Morse Code. This sequence is known as Rule 14(l) " Rule # 14(l) – Approaching public crossings at grade, to be prolonged or repeated until crossing is reached unless otherwise provided". This rule is applied in almost all U.S. railroad operating rule books even if the advance warning provided by the horn will be less than 15 seconds in duration. This signal is to be prolonged or repeated until the engine or train occupies the crossing; or, where multiple crossing are involved, until the last crossing is occupied. The same rule is practiced when approaching locations such as rail yards, where men may be working on the tracks, as well as bridges, tunnels and other points.
Modern whistle posts are of simple sheet metal construction, utilizing a 'W' marker. Some multiple crossings protected by a single sign have an 'X' displayed below the 'W', other multiple crossings may have 2 or 3 W markers on the same post.
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.
United Kingdom
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.
Some lines have other signs. for example the word 'whistle' underneath a triangle with an exclamation mark in it is sometimes used on mineral railways.
Republic of Ireland
In Ireland, a whistle post is an upright rectangle bearing black and yellow diagonal stripes.
France
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.
Germany
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).
Norway
A yellow diamond with black edges, Signal 67, or one of its varieties, is placed 250–500 meter before halts and level crossings where the public is not otherwise warned about approaching trains. The signal is a two- to three-second blast with the horn.
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.
Sweden
In Sweden, a whistle board is a black triangle with a white border, pointing downwards. Whistle boards before level crossings has an additional sign, a yellow square with a black V. If the trains are supposed to sound the horn at daytime only (between 6 a.m. and 10 p.m.) there is another additional sign, rectangular and black/white. For a level crossing, the horn signal is a three-second blast at the whistle board and a three-second blast between the board and the crossing.
Thailand
In Thailand, whistle post consists of circular sign with Thai Character "ว" on it. "ว" comes from the word "หวีด" which means whistle.
There are some variant of the sign. One with thick line below the character is usually installed before the level crossing, another without the line is installed within some yard nearby local villages. Additional sign indicating the type of crossing gate is installed just below the whistle sign.
People's Republic of China
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.