You may have seen one of these whiteboard signs with a W on it – W signs are called whistle posts, and they are usually located at highway-rail grade crossings.
In general, locomotive horns must be be sounded at all public grade crossings at least 15 seconds, but not more than 20 seconds before entering a crossing. This rule applies when the train speed is below 45 mph (70 km/h). At 45 mph or above, trains are still required to sound their horn at the designated location (usually marked with a whistle post).
The pattern for blowing the horn remains two long, one short, and one long, and is to be repeated until the lead locomotive fully occupies the crossing.
And now you know…