Baltimore and Potomac Tunnel

[1] The 7,669-foot (2,338 m) tunnel, which passes under the Baltimore neighborhoods of Bolton Hill, Madison Park, and Upton, consists of a single tube with two tracks.

The railroad lowered the tunnel floor about 2+1⁄2 feet (0.76 m), underpinned the side walls, installed a concrete invert slab, and rebuilt the track structure.

[7] In preparation for the electrified operation, the tunnel was lined with gunite to waterproof the arch and prevent icicles from shorting out the catenary wires.

[7] In the late 1950s, the tunnel became a hindrance to the growth of PRR's Trailer-on-Train service, which required additional vertical and horizontal clearance to accommodate semi-trailers on top of railroad flatcars.

[7] In addition, even with the gauntlet track, the tunnel could not accommodate cars that were taller than 16 feet 3 inches (4.95 m) or exceeded the Plate C loading gauge.

the gauntlet track was removed, due to freight traffic largely shifting to the ex-Baltimore and Ohio route through the Howard Street Tunnel.

[1] On January 28, 2010, $60 million in funding was awarded under the American Recovery and Reinvestment Act to conduct the study, but not the money that would be required for a replacement tunnel.

[19] Highlights of Alternative 3B are: The Federal Railroad Administration released its Record of Decision on the new tunnel, the final step in the NEPA process, in March 2017.

[25] On January 30, 2023, President Joe Biden visited the tunnel to kick off the project and promote the $1 trillion Infrastructure Investment and Jobs Act (IIJA).