They're called commuter trains because they run most often in the morning and the afternoon, when people need to go to and from work. They usually run under the ground. The longest subway in the world is in New York City. It covers 722 miles and takes over 2.7 million New Yorkers to work every day. Heavy rail trains go faster than light rail trains, and cost more to build.
Heavy rail can be one of the most nicest forms of public transit.
The trains are fast. Cars don't get in the way of the heavy rail trains, and the trains don't get in the way of the cars, either. But heavy rail is expensive. Monorails have never gotten very popular, although some people think they should be. If you'd like to ride a monorail, ask your parents to take you to Seattle.