Trains are required to sound a horn when they are approximately:

• 1,500 feet from passing through a railroad crossing
  Explanation:

  "Trains are required to signal a horn when they are approximately 1,500 feet from passing through a railroad crossing."
• 2,000 feet from passing through a railroad crossing
• 2,500 feet from passing through a railroad crossing
• 3,000 feet from passing through a railroad crossing