What is Train horn?

A train horn is an air horn that serves as an audible warning device on diesel and electric powered trains. The horn's primary purpose is to alert persons and animals to an oncoming train, especially when approaching a level crossing. They are often extremely loud, allowing them to be heard from a great distance. The horn is also used for acknowledging signals given by railroad employees, such as during switching operations. For steam locomotives, the equivalent device is a train whistle.