The U.S. Senate website goes into detail about the monorail system:

The distance between the old Senate Office Building and the Capitol was only a fifth of a mile, but senators needed to traverse it multiple times on a typical legislative day. Had the Capitol been a skyscraper, elevators would have whisked members from their offices on different floors to the chamber. Instead, the office building and Capitol were linked by a horizontal elevator: a subway. Initially, transportation in the subway tunnel was provided by battery-powered yellow Studebaker coaches. Ten passengers could ride in each car, facing each other on benches. The buses ran along a concrete roadway at a maximum speed of 12 miles an hour. Rather than turn around at each terminus, they backed up for the return trip. Running backwards made them more difficult to steer, raising fears that someday the two cars might collide in the curved tunnel. In 1912 the Senate authorized installation of a double-line electric monorail system that ran on fixed tracks. Built by the Columbia Construction Company, the trains held up to 18 passengers each, in rows of wicker seats. Senators received priority in boarding the cars, and the front seats were reserved for them at all times. One operator commented that he never waited for other passengers when there was a senator aboard, “unless it’s the Vice President.” Senators summoned the trains by ringing a bell three times (the same signal also alerted elevator operators). During Senate votes, the trains would shuttle back and forth furiously, a one-way trip taking 45 seconds. Each car made an average of 225 trips per day when the Senate was in session. The press dubbed this Senate subway the “shortest and most exclusive railway in the world.”

After the subway was extended to the new office building, a new tunnel and rail system was constructed for the original office building. The first subway tunnel was converted into storage space, offices and facilities for the Senate Recording Studio. The old cars made their last run in 1961, and were captured on film by the Hollywood movie Advise and Consent, shot during the trains’ final weeks in service. The new cars featured upholstered seats and a plastic shield to protect passengers’ hair from becoming windblown. Not everyone appreciated the improved services. Arkansas Senator J. William Fulbright lamented the closing of the old line, which he described as “soothing to jangled nerves.” Senator Fulbright argued that the quaint old cars had put senators “in a friendly and amiable frame of mind as they arrive to do battle over the nation’s business.”