Each ''chug'' is the noise made by escaping steam as the engine's valve gear releases steam at the end of one stroke of one cylinder. For example, in a two-cylinder steam locomotive, the connecting rods turn the driving wheels at one revolution per complete cylinder cycle.

The rhythmic "chugging" or "chuffing" sound associated with trains is primarily a characteristic of steam locomotives. This sound is produced by the rhythmic release of exhaust steam. After steam pushes the pistons to turn the wheels, it is exhausted through a blast pipe and out of the smokestack. Each "chug" corresponds to a stroke of the piston; as the steam is forced out at high pressure, it creates a vacuum that pulls air through the firebox to keep the fire hot. As a locomotive speeds up, the frequency of these exhausts increases, turning the slow "chuff-chuff" into a rapid roar. While modern diesel-electric trains don't "chug" in the same mechanical way, they produce a deep, pulsing thrum from their large multi-cylinder internal combustion engines. This "prime mover" engine runs at a relatively constant speed to generate electricity, creating a heavy, vibrating sound that people often still describe as a "chug" when the train is pulling a heavy load under high power.