1) locomotives and trains only have horns fitted, not a bell.
2) all locomotives and trains in heavy rail use (in the UK that's anything that isn't a tram, which is light rail) must have two, separate, horns, one sounding a low note (Airchime KS1 or equivalent usually) and the other sounding a high note (Airchime KS2 usually).
3) more modern units feature the ability to have a soft and a loud tone for both high and low horns, either through a special four-way valve, or by means of a speed-sensing automatic diverter.
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When to sound horn
- when passing a whistle board - low loud tone once, only between 0600 and 2359.
- warning anyone on or near the line - high loud and low loud tone, repeating if needed.
- urgent warning to anyone on or dangerously near the line - series of short high loud tones.
- before entering or leaving a maintenance shed - low soft tone short.
- warning to anyone while on a depot or siding - low soft tone short.
- when shunting and clear of points with points controlled locally - low or high loud tone, short.
- before passing over a level crossing in the wrong direction - low or high loud tone, long.
- train in distress warning - a continuous series of long blasts on the high loud tone.
- making a movement in the wrong direction - repeated series of high loud short tones.
- failed headlight or broken windscreen - sound horn more frequently than usual.
- when making a movement within an engineering possession - low loud short.
- local or special code - high loud tone.
- warning a driver of an oncoming train of a hazard
- locally monitored level crossings switched off or failed - low or high loud tone, long.
- at any other time thought necessary by the driver.
Faults with the horn
If one tone (either low or high) is defective or failed, a train can start or finish a journey (but not from maintenance depot).
If both tones have failed, a train must not exceed 20mph and will be taken out of service as soon as possible.
