Why do trains whistle at crossings?

Answer: Warn vehicles and pedestrians

Warn vehicles and pedestrians ✓ — Correct! Train whistles (horns) are legally required at road crossings to warn vehicles and pedestrians. Trains can't stop quickly (a loaded freight train may need over a mile to stop), so advance warning through whistle is critical for safety. The whistle pattern varies by situation.

Test the whistle mechanism — Wrong. Whistles aren't tested at every crossing. The whistle is a legal safety warning to road users approaching the crossing.

Traditional railroad custom — Wrong. While traditional, crossing whistles are mandatory safety warnings, not just custom. They alert road users who might not see or hear the approaching train.