The power output of a train varies wildly depending on its type and purpose, but it is typically measured in kilowatts (kW) or horsepower. A modern high-speed electric train, such as the French TGV or the Japanese Shinkansen, can have a power output of around 8,000 to 12,000 kW (roughly 10,000 to 16,000 horsepower). To put that in perspective, 12,000 kW is 12 million watts, which is enough to power thousands of homes simultaneously. Heavy-duty freight locomotives, like the ones used by Union Pacific in the U.S., typically produce between 3,000 and 4,500 horsepower (approx. 2,200 to 3,300 kW) per locomotive unit, and multiple units are often linked together to pull a single long train. Smaller commuter or light-rail trains might only require 500 to 1,500 kW. The efficiency of electric trains is remarkably high because they receive a constant supply of power from overhead lines (catenary) or a third rail, allowing them to accelerate rapidly and maintain high speeds with a much lower environmental impact compared to diesel-electric locomotives, which must generate their own electricity on board using a massive internal combustion engine.