A subway line is an underground or elevated train system designed to transport passengers quickly across urban areas. It consists of a series of interconnected stations where trains stop to pick up and drop off riders. Subway lines are typically part of a larger public transportation network, which may include buses and trams. Subway systems are known for their efficiency and ability to reduce traffic congestion. They operate on fixed schedules and can carry large numbers of passengers simultaneously. Major cities like New York City, London, and Tokyo have extensive subway networks that serve millions of commuters daily.