A train horn is a loud sound device used on trains to alert people and vehicles of the train's approach. It is designed to be heard over long distances and is crucial for safety at crossings and in populated areas. The horn typically produces a series of distinct notes, which can vary by train type and manufacturer. Train horns are regulated by law, requiring specific sound levels and patterns to ensure they are effective. The sound is created using compressed air, which is released through a series of pipes and valves. This mechanism allows the horn to produce a powerful and attention-grabbing noise, essential for preventing accidents near railroad crossings and train stations.