As of 2020, there were six main lines on the system, two of which only run during weekdays. There are of track (using a wider-than-normal gauge of ). There are 50 stations. Like other rapid transit systems in the world, tracks are in underground tunnels in inner cities, and on elevated viaducts in suburban areas. In some places, they are in the middle of freeways. The busiest part of the system is the Transbay Tube, a underwater tunnel under the San Francisco Bay. Almost 150,000 people pass through the tube every day.