The main reason why diesel locomotives are hybrid is because this combination eliminates the need for a mechanical transmission, as found in cars.
The giant two-stroke, turbocharged engine and electrical generator provide the huge amount of power needed to pull heavy loads at high speeds. Cummins' locomotive engine weighs over 24,000 pounds (10,886 kilograms).
So why two-stroke? Even though this engine is huge, if it operated on the four-stroke diesel cycle, like most smaller diesel engines do, it would only make about half the power. This is because with the two-stroke cycle, there are twice as many combustion events (which produce the power) per revolution.
The traction motors provide propulsion power to the wheels. There is one on each axle. Each motor drives a small gear, which meshes with a larger gear on the axle shaft.
