In 1866, the B&P sought permission to build a branch into Washington from a point within 2 miles of the Collington (now Bowie) Post Office in Prince George's County and also signed a contract to begin construction of the main line between Baltimore and the Potomac.

[6][7] The B&P was working with the Pennsylvania Railroad (PRR) and its ally, the Northern Central Railway (NCRY), which wanted its own route to Washington, DC and Virginia.

[12] By the end of that year, the right-of-way had been graded from the Patapsco to the District Line and from Bowie to Upper Marlboro; and several small bridges had been constructed.

[13] In 1870, the B&P was granted permission by Congress to extend their Washington branch across the Potomac River Railroad Bridge to Virginia, if they would maintain it.

By the summer of 1870, work was underway on both the Virginia Avenue Tunnel and the Anacostia Railroad Bridge; and by the fall track was being laid in a few places.

By September 1871, The B&P was actively building bridges across the Potomac, the Patapsco, Gwynn Falls and Anacostia; and working on the tunnels in DC and Baltimore.

[22] By late November the bridges over Beaver Dam and Watts Branch were complete and track had been extended to the Anacostia.

[29] Around the same time, Congress granted the B&P permission to build a depot at the corner of 6th and B (Now Constitution) NW on a site previously reserved for a park.

[30] The final spike for the "Main Line" to Pope's Creek was driven on June 10, 1872, but trains did not start running yet.

[35][36] The line to Pope's Creek was finished in late December, formally opened on January 1, 1873, and the first trains were run the next day.

[46][page needed] The station was built over the old Washington City Canal, which complicated the construction of the foundation.

[4]: 340  Tracks ran south from the station along Sixth Street to a wye junction at Sixth Street SW, Maryland Avenue SW, and Virginia Avenue SW. On the morning of July 2, 1881, U.S. President James A. Garfield was shot in the waiting room of the B&P station in Washington, D.C.[47] Although the shot was not fatal, he died in September 1881 as a result of infections from the injury.

[51] In the case of the Washington Branch, MARC took over commuter rail service but did not buy the line as in 1981 Amtrak had bought it.

The main line from Bowie to Washington, a distance of 17.1 miles (27.5 km), was provided for in the charter as a branch.

The Mechanicsville line was eventually extended to the Patuxent River Naval Air Station at Cedar Point.