This line was the only route west out of Baltimore until the Metropolitan Branch was constructed from Washington, DC to Point of Rocks in the 1870s.

For instance, the route was laid out to minimize grades at the expense of curvature; over the next century, however, to eliminate and bypass the sharp curves that resulted from this decision, bridges and tunnels were constructed.

The planes over Parr's Ridge also resulted from this same thinking, and subsequently gained the distinction of becoming one of the first railroad main line right-of-way abandonments in history.

[4] In the 1850s, when Benjamin Henry Latrobe, II was chief operating engineer, the need to address these deficiencies became acute, and a variety of improvements were made, subject to the railroad's limited resources at the time.

Among these was the "Elysville cutoff," where a pair of bridges were constructed to bypass a sharp curve on the south side of the river.

Most of these were designed by E. Francis Baldwin and several towns on the Old Main Line received such stations, erected in either wood or brick.

[6][7] At the west end of the line, the Adamstown Cutoff was built to allow operation of coal drags over the OML, minimizing the use of helpers.

During World War II, however, traffic rose dramatically, and a new water and coal station was added at Gaither to allow engines to be serviced away from the congestion of Baltimore City.

In 1959, the line was reduced to single track to increase the clearance through the tunnels, and Centralized Traffic Control (CTC) was introduced.

The service, which branches from the Brunswick Line at Point of Rocks, was started in response to the substantial growth of commuters between Frederick and Washington during the 1990s.

Prior to the start of the service, a leg was added to the wye between the OML and the Metropolitan Branch at Point of Rocks to allow trains traveling between Frederick and Washington to make a direct movement between the two lines.