Shepherd's whistle
They are also produced commercially. Simple whistles may cost less than 1 USD in bulk; expensive ones may be quite ornate. They are commonly made of plastic, aluminum, stainless steel, silver, brass, titanium, Corian, jade, buffalo horn and other materials.
Train whistle
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.