New B&O president Charles F. Mayer made the Belt Line a priority by the 1870s, and recruited young railroader Samuel Rea to work on it as Chief Engineer.

Building around the outskirts of town would have required massive regrading and bridging, as the terrain is extremely hilly and the line would cut across every watershed flowing into the harbor.

The track then curved around the northwest corner of the center city going east, passed through six other (much shorter) tunnels, continuing across the northern outskirts of downtown, curving around the Northeast corner of the old City, finally heading southeast to meet the already constructed line just north of the Canton neighborhood and connecting with the East Coast route.

Due to the built-up nature of the area traversed and the hilly terrain, much of its line through town was in tunnels, which posed severe ventilation problems.

Large chimneys were constructed above the Pennsylvania line, in a not entirely successful attempt to disperse the fumes from the coal-fired locomotives.

[5] This equipment was delivered beginning in 1895, and the first train pulled by an electric locomotive operated through the Howard Street Tunnel on June 27, 1895.

Northbound freight trains were pulled by electric locomotives for another two miles, until reaching Huntingdon Avenue in east Baltimore.

When the Howard Street track was made into a gantlet to allow higher clearances, the pickup contacts on one side were mounted on swinging arms to accommodate the varying distance to the third rail.

Freight traffic was also limited by the scarcity of online industry east of Baltimore and B&O's lack of a connection across the Potomac River at Washington, D. C., to the southern railroads.

This segment of the CSX network is considered a chokepoint for freight train service from the Port of Baltimore to East Coast and Midwest markets.

On July 18, 2001, a 60-car CSXT freight derailed in the Howard Street Tunnel, sparking a fire that burned for six days and blocked traffic for much longer.

The Howard Street Tunnel fire called attention to the Belt Line, both as a risk to the surrounding structures and as a link in rail traffic.

CSXT has implemented various improvements to increase the integrity of the link, but is limited by the shallow depth of the bore (only three feet or 0.91 meters below the surface at the south end) and the instability of the surrounding soil.

On April 30, 2014, a block-long portion of a retaining wall in a below-grade stretch of the Belt Line in southern Charles Village collapsed after a heavy rainstorm, sending part of East 26th Street and a number of cars onto the tracks.

[16][17][18][19][20] On July 22, 2019, U.S. Transportation Secretary Elaine Chao announced the award of $125 million in federal infrastructure funding for the project.