多选题 You"re more likely to see a tavern or restaurant named the Whistle Post than to notice the little signs along the tracks marked with a W or an X. The whistle-post signs tell train engineers where to begin blowing a warning prior to crossing an intersection. They are especially useful where the railroad crossing has no automatic barrier—on rarely traveled crossings or when approaching rail yards, for example. Train whistles have largely been replaced by air horns, but the rules of rail travel still call for two short, one long, and one short blast of the horn to be repeated until the train reaches the crossing. The noise may be a nuisance, but studies have shown that accidents double when the horn is not used.
Based on the information in the passage, which of these can be inferred about whistle posts?