A horn is a warning signal, not just a volume knob
A train horn has to stand out against traffic, wind, machinery, and other outdoor noise. Making a sound louder helps, but a distinctive pattern of pitches can make it easier to recognize as a train warning. A siren usually sweeps up and down in pitch to signal urgency; a train horn typically uses a steady blast that can be heard as a single, unmistakable call.
How several notes become one sound
Many train horns contain several individual air-powered horns, each tuned to a different pitch. When they sound together, those notes form a chord: the combined sound has more tonal richness than one horn alone. Their pitches are chosen and aligned so the chord sounds coherent rather than like a jumble of competing tones. The result is not a melody—the notes are heard together as one warning.
The practical payoff
A tuned chord gives the horn a recognizable character while keeping its pitch steady during a blast. That matters because a warning must be noticed quickly, not interpreted as a changing signal. The exact combination is not universal; horns vary by train and design. But the underlying idea is practical: use several carefully matched tones to make a powerful warning sound distinctive and coherent.
