Choo-choo was the noise made by steam leaving the cylinders, which were mounted down by the wheels. If you've never seen that in real life, you have seen it in movies. A conductor shouts, All aboard, steam gushes about the wheels, and the train starts to move.

The iconic "choo choo" sound is a hallmark of steam locomotives and is caused by the release of spent steam into the atmosphere. In a steam engine, high-pressure steam is directed into cylinders to push pistons, which in turn move the wheels. Once the steam has done its work of pushing the piston to one end, it must be exhausted to make room for fresh, hot steam. This "spent" steam is blasted out through a blast pipe and up the smokestack, creating a rhythmic chuffing noise. Because most locomotives have two cylinders operating out of phase, you hear four "chuffs" for every full rotation of the wheels. The intensity and pitch of the sound change as the train gains speed or works harder against a grade, transforming the "choo choo" into a powerful roar that symbolized the industrial era's might.