Early practice often saw single note whistles supplied for freight engines, and 3-chime for passenger. This practice carried to diesel, with some railroads putting 5-chimes mostly on passenger, leaving 3-chimes or single honks for freight/switch.
Airchime's catalog suggests this arrangement. The primary purpose for multiple tones is to keep the whistle/horn from being drowned out by background noise of a similar pitch (especially these days).