Diesel-electric locomotives are a type of train engine that combines a diesel engine with an electric generator. The diesel engine powers the generator, which produces electricity to drive electric traction motors. This setup allows for efficient power transfer and better control of the locomotive's speed and torque. These locomotives are commonly used in freight and passenger services due to their versatility and ability to operate on tracks without the need for external power sources. They are often preferred for their lower emissions compared to traditional steam engines, making them a more environmentally friendly option in the rail industry.