from my driver's safety course:

Why should you stop 15 feet from the tracks?
1. A person or signal warns that a train is coming.
2. You see a train coming.
3. You hear the horn or bell of a train close by.
4. Section 316.1575, Florida Statutes requires you to stop.

Disregarding the odd phrasing of the "question", I do have to say I really enjoy reason number 4. Why? BECAUSE WE SAID SO!