from coal smoke, and after a short time it was replaced by a conventional third rail system. 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. The electrification was finally discontinued in 1952 when dieselization made it unnecessary. Latter day operation Throughout much of its history until the end of passenger trains in 1958, the line had relatively low passenger traffic, averaging six daily New York–Washington trains each way. 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. The massive Pennsylvania main line carried most traffic to the northeast. By the 1970s, however, both railroads were failing financially. The PRR had been merged into Penn Central in 1968, and two years later the new company declared bankruptcy. The B&O became part of the Chessie System in 1973, which in turn was merged into CSX in 1980. In 1976 the PRR line became part of the newly formed Amtrak system. The line was subsequently called the Northeast Corridor (NEC), and its role as a freight line became relatively minor. The 1987 accident at Chase, Maryland, involving a Conrail locomotive led to further reductions in freight traffic on the NEC. The Belt Line (now operated by CSX) is therefore a key link in what has now become the principal rail freight line from Baltimore to Philadelphia and beyond. 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. See Baltimore Terminal Subdivision. Howard Street Tunnel fire 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 below the surface at the south end) and the instability of the surrounding soil. Retaining wall collapse On April 30, 2014, a block-long portion of a