The equivalent of a horn on a car, a train whistle is an important device for safety reasons. Originally train whistles were steam powered like the locomotives they were on; on modern railroad engines, however, they are usually extremely loud air horns. The main purpose of the train whistle is the enhancement of safety by alerting those in the area that the train is approaching, particularly near crossings. They are used to warn motor vehicles and pedestrians as well as to signal to other trains and railroad employees. Many of the horns have a distinctive tone to the sound they emit, and some hobbyists can tell which train is coming just by listening.
The first trains were steam powered locomotives and did not have whistles, although some were equipped with bells. These bells could not be heard very well over the sound of the train itself, and the sound did not travel far. The steam powered train whistle was developed in England in response to a collision between two locomotives, and its use eventually was adopted throughout the world. The whistles were originally mounted above the boiler and used the same steam to produce sound that powered the train. The loud piercing sound they produced could be heard from a great distance and were instrumental in reducing the frequency of train accidents.
When steam locomotives were replaced by diesel, the train whistle had to evolve along with the change. Steam powered whistles were replaced by large air horns. These produce sound with compressed air that travels through a chamber in the horn and vibrates a flexible diaphragm. The vibration of the diaphragm against a nozzle creates an extremely loud sound that can be heard from far away.
The primary purpose of the train whistle is to enhance the safety of those in the area surrounding the railroad tracks. Whenever the train approaches a crossing or a highly populated area, the whistle sounds to alert nearby people and animals, as well as motorists, that it is coming. The horns are used even when a crossing has gates as an added layer of protection. Another purpose for the train whistle is signaling. The whistles signal other trains and to acknowledge directions from railroad employees during switching maneuvers.
There are several different manufacturers of train engines, and each use a slightly different train whistle. Each type produces a distinct sound, and enthusiasts and hobbyists can distinguish among them via the whistle. Some hobbyists also collect train whistles along with other railroad paraphernalia.