The Steam Whistle can be placed on Fluid Tanks. The Tank must have a heat source beneath it for the Whistle to function. It can be activated by Redstone. Right click with a Whistle in hand to extend it. The Whistle can be extended up to 12 times, playing up to 13 notes per octave. Use a Wrench to change the octave; 3 octaves are available. Engineer's Goggles can be used to view the current note.
Steam Whistles will also function on Trains, complete with a different set of sounds. While driving the train, use the sprint key to blow the whistle. The Whistle's pitch will be accounted for. Non-player conductors will also blow the whistles periodically between distant stations.
