The U.S. Capitol subway consists of three lines: two on the Senate (north) side, and one on the House (south) side.

The Architect of the Capitol (AOC) is responsible for the operation and maintenance of the Capitol "subway system." 

The purpose of the subway is to allow for safe and expedient travel for Senator and Members of Congress to travel from the office buildings to their respective chambers within the Capitol for voting.

The original subway line was built in 1909 to link the Russell Senate Office Building to the Capitol serviced by cars built by the Studebaker Company. They were first put into service on March 7, 1909, the day the Senate Subway opened. This was replaced in 1912 by a monorail vehicle which featured a wicker coach.

In 1960, an operator-controlled monorail was installed for the Dirksen Senate Office Building. A two-car subway line connecting the Rayburn House Office Building to the Capitol was built in 1965. The Dirksen monorail, which had been extended to the Hart Senate Office Building in 1982, was replaced in 1993 by an automatic train.

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The original subway line was built in 1909 to link the Russell Senate Office Building to the Capitol serviced by cars built by the Studebaker Company.
The original subway line was built in 1909 to link the Russell Senate Office Building to the Capitol serviced by cars built by the Studebaker Company.