The trains from the Russell Senate office building and the Rayburn House office building each have conductors, while the monorail that links the Dirksen and Hart Senate office buildings run on an automatic system.
While mostly frequented by lawmakers, the trains are for anyone to ride as long as they are cleared to enter the Capitol.
The subway system underneath the Capitol opened in 1909 as a way for lawmakers to get to and from their offices and the House and Senate chambers.
The original version used Studebaker cars. They were replaced by a monorail in 1912.
The Dirksen Senate Office Building got an operator-controlled monorail in 1960, and a line connecting the Rayburn House Office Building to the Capitol opened five years later.
Today, the subway system consists of three lines. Two run along the Senate side of the Capitol, on the north side, and one runs along the House side to the south.
Many different lawmakers, including future presidents and other high-profile politicians, have used the Capitol subway system as part of their daily routines in Congress.
The train that connects the Rayburn building to the House side of the Capitol has one car that is only for members of Congress, while the other cars on the track can be used by anyone.
Tom Williams/CQ Roll Call
The other trains in the Capitol subway system do not have "members only" cars.
One side of the subway system is open to the public, meaning anyone who enters the Capitol Hill office buildings can see who comes and goes, including protesters looking to heckle lawmakers during tense times.
Tom Williams/CQ Roll Call
But if individuals are not cleared to go into the Capitol, they will not be allowed to advance past the heavy police presence.
Most of the traffic through the tunnels include lawmakers, their staff, and credentialed members of the press.
When they are in the mood, senators will walk and talk with reporters while riding up the escalator into the Senate.
Right where the Senate trains end, there is an escalator to take them up to the elevators where they will then go up to the chamber. Reporters will often jog up the staircase alongside the escalator as senators will take questions.
Some lawmakers prefer the subways, while others almost always walk the long length of hallways. Republican Sen. John Cornyn of Texas is known to take a group of reporters with him to discuss policy and strategy as he speed-walks the nearly half-mile back to his office in the Hart building.
Tom Williams/CQ Roll Call
Former Tennessee Sen. Bob Corker regularly engaged with the media during his time in the Senate. Once he would step off the train, he would walk and talk with reporters, often holding a lidless cup of scalding hot coffee.
When senators are in the mood to go further in depth about whatever policy or situation they are discussing at the moment, they will sometimes take a journalist or a few with them for the train ride.
It doesn't happen too often, but when it does, it can produce some insightful context on key issues.
Sometimes the lawmakers just want to be left alone. Because the tunnel system on the Senate side bottlenecks into a small area where dozens of journalists will congregate, some members will often sprint from the elevators to the subway to avoid getting pegged with questions.
AP Photo/Pablo Martinez Monsivais
One method senators have developed since the invention of the mobile phone is making sure to take a call or sometimes fabricate one to avoid getting tough questions from reporters in the Senate subway area.
While it is mostly lawmakers, staffers, and members of the media taking the trains, anyone allowed into the Capitol can hitch a ride.
Sometimes celebrity guests (like Denzel Washington!) will make appearances if they have meetings with members of leadership or want to visit the inside area.
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