They run on electricity from power lines over the train tracks. They can go as fast as fifty-five miles an hour, although they usually go much slower in the city. Light rail lines are more expensive than buses, but cheaper than heavy rail lines. The Muni trains in San Francisco, the Portland MAX and the San Diego Trolley are light rail lines. So is the Los Angeles Blue Line, shown in the photo below.
Some commuter trains run on electricity too.
Others are powered by locomotives, just like the trains that transported people and products around the country before cars were invented. This engine is much, much bigger and more powerful than the engine in any car. It makes three thousand horsepower and can pull a nine hundred thousand pound train over one hundred miles per hour. Commuter trains often are nicer inside than most other types of trains. They sometimes have bathrooms, and big, comfortable chairs, and tables, so business people can do work while they're riding to the office.