Washington, D.C., has a population of approx. 600,000, although with some 3.9 million in the metropolitan area, which extends into their neighbouring states of Maryland and Virginia. Compared to older subways, Washington's underground stations feature spacious vaulted roofs. Most outer branches run on the surface, the Orange Line towards Vienna runs in the median strip of an expressway. Apart from the Red Line, all other four lines form an interlaced network with two lines sharing tracks and platforms along certain stretches.
Currently the system includes 171 km (85 km underground) and 90 stations (52 underground).