Baltimore and Potomac Tunnel
The Baltimore and Potomac Tunnel (or B&P Tunnel) is a double track, masonry arch railroad tunnel beneath Baltimore, Maryland. It now serves Northeast Corridor rail service operated by Amtrak and MARC Train passenger railroads with an average of 135 trains per weekday traversing the tunnel.
Whether considered a single 7,669-foot (2,338 m) bore punctuated by two short open cuts, or thought of as a tunnel with three sections, the facility is collectively referred to as the B&P Tunnel and constitutes the southern approach to Pennsylvania Station. It passes under the Baltimore neighborhoods of Bolton Hill, Madison Park and Upton, and is a bottleneck for rail traffic along the Northeast Corridor. A sharp curve at the south portal of the tunnel prevents southbound trains from exceeding 30 mph (48 km/h) while in the tunnel.
Construction and modifications
Constructed by the Baltimore and Potomac Railroad under Winchester Street and Wilson Street in Baltimore, the tunnel opened on June 29, 1873. The B&P tunnel allowed the Pennsylvania Railroad (PRR) direct access to Washington, D.C. for the first time by connecting its Northern Central Railway affiliate (which arrived in Baltimore from the north) to the Baltimore and Potomac's new spur, which ran to Washington.
Between 1916 and 1917, the PRR lowered the floor of the tunnel approximately 2½ feet to accommodate larger trains. The work included the underpinning of the side walls, installation of a concrete invert slab, and reconstruction of the track structure. The bases of the tunnel walls were chipped away to improve horizontal clearance.
Prior to the electrification of the PRR's New York City to Washington main line in 1935, the poorly-ventilated tunnel easily filled with smoke from the steam locomotives then in use. The smoke also was a nuisance to the residential neighborhoods above the tunnel.
The tunnel was lined with gunite to waterproof the arch and prevent icicles from shorting out the catenary wires prior to the initiation of electrified operation. However, financial considerations prevented the PRR from constructing a new passenger tunnel on the Presstman Street alignment, for which it previously had acquired rights. The PRR’s plan had envisioned using the new Presstman Street tunnel and the original bores of the Union Tunnel for passenger operations, while the old B&P Tunnel and the newer bores of the Union Tunnel (completed in the 1930s) would have been used for freight operations.
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. The curve at Pennsylvania Avenue was the biggest constraint. The PRR modified the tunnel walls and ceiling for a distance of 2,200 feet (670 m) to improve clearance and enable high cars and piggyback trailers to traverse the tunnel without damaging their roofs. Additionally, a 928-foot (283 m) long gantlet track was installed on southbound track 3 to route trains 17 inches (430 mm) closer to the middle of the tunnel. However, trains could not operate on track 2 while track 3 and the gantlet were being used. The gantlet track effectively created a single-track tunnel when in use; if a freight train broke down while using the gantlet, the tunnel was closed to traffic until the train was moved.
The tunnel underwent rehabilitation as part of the Northeast Corridor Improvement Project in the early 1980s. The repairs included replacing the existing invert, repairing the tunnel lining, upgrading the track structure, installing a new gantlet track, and rehabilitating the tunnel drainage system. No fundamental change, however, was made in the tunnel’s difficult geometry. Eventually, the gantlet track was removed due to changes in freight traffic patterns. 
In June 2008, the U.S. House of Representatives voted to support a study of the environmental impacts of different possible replacement tunnels. 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 will be required for a replacement tunnel. 2013 estimates put the cost around $1.5 billion.
The tunnel's height, speed, and capacity limitations would threaten the ability to be competitive for increased shipping volume at the Port of Baltimore.
- Brown, Matthew Hay (2008-06-12). "House OKs funds for tunnel study: Alternative sought to outmoded passage that runs under city". The Baltimore Sun.
- U.S. Federal Railroad Administration. Washington, DC. "Report To Congress: Baltimore's Railroad Network, Challenges and Alternatives." November 2005. p. 2.16.
- Wilson, William Bender (1895). History of the Pennsylvania Railroad Company: With Plan of Organization. Philadelphia: Henry T. Coates. p. 339.
- Robert T. Netzlof (2002-06-12). "Corporate Genealogy Union Railroad". Retrieved 2007-10-01.
- "Mikulski, Cardin Laud Federal Investment in High-Speed Rail for Maryland" (Press release). U.S. Senator Barbara Mikulski. 2010-01-28.
- Rector, Kevin (22 November 2013). "Aged tunnel where Amtrak train derailed may be replaced". The Baltimore Sun. Retrieved 25 November 2013.
- U.S. Federal Railroad Administration. Report To Congress: Baltimore's Railroad Network, Challenges and Alternatives. November 2005.