A train whistle or air whistle (originally referred to as a steam trumpet) is an audible signaling device on a steam locomotive, used to warn that the train is approaching, and to communicate with rail workers. Modern diesel and electric locomotives primarily use a powerful air horn instead of a whistle as an audible warning device. However, the word whistle continues to be used by railroaders in referring to such signaling practices as "whistling off" (sounding the horn when a train gets underway).
The need for a whistle on a locomotive exists because trains move on fixed rails and thus are uniquely susceptible to collision. This susceptibility is exacerbated by a train's enormous weight and inertia, which make it difficult to quickly stop when encountering an obstacle. Hence a means of warning others of the approach of a train from a distance is necessary. As train whistles are inexpensive compared to other warning devices, the use of loud and distinct whistles became the preferred solution for railway operators.
Steam whistles were almost always actuated with a pull cord (or sometimes a lever) that permitted proportional (tracker) action, so that some form of "expression" could be put into the sound. Many locomotive operators would have their own style of blowing the whistle, and it was often apparent who was operating the locomotive by the sound. Modern locomotives often make use of a push button switch to operate the air horn, eliminating any possibility of altering the horn's volume or pitch.
John Holliday describes the history of train whistles as originating in 1832, when a stationmaster suggested, at the opening of the Leicester and Swannington Railway, that the trains should have an audible signaling device. A local musical instrument builder was commissioned to provide a steam-powered whistle, then known as a "steam trumpet".
The article also describes a train collision with a cart, where the driver had blown a horn (steam whistles having not as yet been invented). One account states that Weatherburn, the engine driver, had "mouthblown his horn" at the crossing in an attempt to prevent the accident, but that no attention had been paid to this audible warning, perhaps because it had not been heard. Although nobody was injured, the accident was deemed serious enough to warrant George Stephenson's personal intervention. Stephenson subsequently called a meeting of directors and accepted the suggestion of the company manager, Ashlin Bagster, that a horn or whistle which could be activated by steam should be constructed and fixed to the locomotives. Stephenson later visited a musical instrument maker in Duke Street in Leicester, who, on Stephenson's instructions, constructed a "steam trumpet", which was tested in the presence of the Board of Directors ten days later.
Stephenson mounted the whistle on the top of the boiler's steam dome, which delivers dry steam to the cylinders for locomotion. The device was apparently about 18 inches (460 mm) high and had an ever-widening trumpet shape with a 6-inch (150 mm) diameter at its top or mouth. The company went on to mount similar devices on its other locomotives.
There is another account that sets the invention of the steam whistle against the actual opening of the line in 1832, rather than associating it with a specific incident.
North American steam locomotive whistles have different sounds from one another. They come in many forms, from tiny little single-note shriekers (called "banshees" on the Pennsylvania Railroad) to larger plain whistles with deeper tones (a deep, plain train whistle is the "hooter" of the Norfolk & Western, used on their A- and Y-class Mallet locomotives). Even more well known were the multi-chime train whistles. Nathan of New York copied and improved Casey Jones's boiler-tube chime whistle by casting the six chambers into a single bell, with open "steps" on top to save on casting. This whistle is still considered the "king of train whistles". It is the most copied train whistle in the United States, and many railroads' shops cast their own version of it.
Another very popular American train whistle was, again, a Nathan product. This was a five-note whistle, with a much shorter bell, and therefore, much higher in pitch. This whistle produced a bright G-major 6th chord (GBDEG) and, again, was heavily imitated, copies being made by many different railroads.
The most popular American chime train whistle was the three-note version. These were either commercially made (Crosby, Lunkenheimer, Star Brass, Hancock Inspirator Co. among others) or shop-made by the railroads themselves. Some famous and very melodious shop-made train whistles were Pennsy's passenger chimes and the Baltimore and Ohio's step-top three chimes. But the most beloved of all three-chime train whistles to the public and railroaders alike were the deep-chorded "steamboat minor" long-bells. A well known commercially made chime was Hancock Inspirator Company's three-note step top. These found use on almost every American railroad. Some railroads copied these also, examples being found on the old St. Louis-San Francisco Railway and Illinois Central.
The Southern Railway made three-chime train whistles. These were all distinctive, having top-mounted levers. They had short-bell three-chimes as well as their (highly copied) long-bell three-chimes on passenger engines, especially their PS4 engines, one of which resides today at the Smithsonian Institution.
Two-note and four-note train whistles never caught on with North American railroads, with one exception: Canadian National Railway created a large four-chime step-top whistle for limited use on some of their locomotives. These were not common and only a few survive today in the hands of collectors. Otherwise, North American train whistles were of the single-note, three-note, five-note and six-note varieties.
These are a few American railroads with whistles valued by collectors:
In the United Kingdom, it is normal for diesel and electric multiple-units and locomotives to have two horns, of different pitches (rather like two-tone emergency road vehicles--police cars, etc.). This has given rise to drivers "playing" unofficial combinations of low and high notes. When passing through the local station in the Yorkshire town of Ilkley, drivers soon began to play the first line of the chorus of the folk song, "On Ilkla Moor Baht 'at" on their horns, using a series of short blasts: low, h-i-g-h high high, low, high, until the practice was stopped by authorities.
The first four notes of Beethoven's Fifth Symphony--played during World War II as the Morse code V (for Victory)--can be sounded on a train horn as three short notes and a longer one, often the last note on the lower-tone horn.
Early railways, before continuous brakes, had the communication chain or cord from the carriages connected to a "brake whistle" on the engine. This was usually of a lower note than the normal whistle used by the driver.
Engines of Britain's Great Western Railway carried two whistles, one low- and one high-pitched. The high-pitched whistle was for warning of the trains approach and for giving shunting signals. The low-pitched whistle was for sending braking instructions to the crew on the train before the advent of continuous brakes and was retained for the same purpose for goods operations. Some whistle-signals required use of both whistles. Some Great Western "autocoaches"--from where the driver operated the steam engine's regulator and brakes, when the engine was propelling one or more autocoaches--still had a whistle connection with the engine's brake whistle, although a gong (much like a tram gong) was fitted at the front of each autocoach and was operated by the driver using a foot treadle.
Back in the days of steam, when assisting engines pushed long goods trains up steep gradients (or "banks"), the train would come to a halt at the bottom of the bank. The assisting engine--or "banker"--would either be attached to the rear of the train, or just come up against the guard's brake van's buffers. Then the banker's driver would whistle--using a series of long blasts and shorts. This told both the signalman and the driver of the train engine that he was ready. The train engine's driver would reply in similar fashion and, with signals at clear, they would set off in unison. If the banker was coupled to the train, when it reached the top of the bank, the train would stop or come to a crawl for the banker to be uncoupled; if not, the banker's driver would just ease off the regulator, allowing the train to continue on its way, with, of course, a whistled "goodbye".
It is not uncommon for the sound of a train's whistle to propagate for miles; yet vehicle operators still have a difficult time hearing the warning signal due to the vehicle's soundproofing and ambient noise within the cab (such as engine, road, radio, and conversation noises).
The need to blare a train's whistle loudly to be heard by the driver of a vehicle approaching a grade crossing has become a major disadvantage to the use of train whistles as a safety device and has caused much controversy among those living within earshot of the train's whistle. It has been documented that a train's whistle, when operating on compressed air, driving an exponential horn, has been measured at a higher decibel levels within the homes of nearby residents than within the cab of a vehicle sitting at the grade crossing.
Given the tonal design of the train whistle, the sound level, how often trains pass through a given community, the number of grade crossings in proximity, and the time of day (night) of occurrence, community residents residing near crossing sometimes feel that train whistles have a serious detrimental effect on the quality of life despite the gain in safety that sounding the horn provides to motorists and pedestrians. However, one Federal Railroad Administration study has shown that the frequency of grade crossing accidents increases in areas where quiet zones are in effect. The study fails to account for other factors that were also introduced at the same time which may have also accounted for the reduction in accidents during the same period the study measured. For instance, it was during the same period that locomotives began sporting the now crucially important tri-lamp headlight arrangement ("ditch lights") and reflector strips similar to those commonly found on highway tractor-trailers. Additionally, the measurements were based on accidents at grade-crossings, which are very low numbers overall to begin with. A grade-crossing that had two accidents during the comparison years, when contrasted with only one accident during the control period, would statistically yield a high percentage-wise improvement in safety, when in reality, it was the difference in only one accident for that grade-crossing.
Conversely, there are those who do not object to the train whistle, as they believe it provides an important safety feature. Some people even like the sound of the whistle, as it calls to mind a nostalgic era, as with the riverboats and their steam whistles and calliopes. However, no real studies have been performed by unbiased official entities to measure the real effects such noise has on a community.
The example of the change in frequency in the whistle of a passing train is often used to explain the phenomena of the Doppler effect. It became a common classroom example after the introduction of trains, since at the time they were one of the few objects that moved quickly while sounding a relatively constant note.
Train whistles are used to communicate with other railroad workers on a train or in the yard. Different combinations of long and short whistles each have their own meaning. They are used to pass instructions, as a safety signal, and to warn of impending movements of a train. Despite the advent of modern radio communication, many of these whistle signals are still used today. (See also Train horn (Common horn signals).)
Signals illustrated below are for North American railroads, "o" for short sounds, and "-" for longer sounds.
Not all railroads use exactly the same whistle signals or assign the same meanings. Some railroads will use their own variations of the above. A few of the signals are obsolete because the workers they were used to communicate with (such as flagman) are now obsolete.
In Norway, for example, the following whistle signals are used:
In Finland, the following are some of the signals used: