Trains are pulled by powerful engine units called locomotives.
In a steam locomotive, pressure from the steam moves a piston back and forth, which turns the wheels via a connecting rod.
In a diesel-electric locomotive, air is drawn into the engine cylinders and is compressed to increase its temperatures; a small amount of diesel fuel is then injected into it. The resulting combustion drives the generator to produce electricity, which is fed to electric motors.
In electric locomotives, electric current is picked up either from a catenary (overhead cable) via a pantograph, or from a third rail.
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