Operation
The whistle consists of the following main parts, as seen on the drawing: the whistle bell (1), the steam orifice or aperture (2), and the valve (9).
When the lever (10) is pulled (usually via a pull cord), the valve opens and lets the steam escape through the orifice. The steam will alternately compress and rarefy in the bell, creating the sound. The pitch, or tone, is dependent on the length of the bell; and also how far the operator has opened the valve. Some locomotive engineers invented their own style of whistling.
Read more about this topic: Steam Whistle
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