D.C. Code § 9-324
Any street railway company using the bridge constructed to replace the Calvert Street Bridge over Rock Creek, as authorized by the Act of June 16, 1933 (48 Stat. 229, ch. 93) shall install thereon, at its own expense, an approved underground system of streetcar propulsion and, at its own expense, shall thereafter maintain such underground construction, and bear the cost of surfacing and resurfacing and maintaining in good condition the space between the railway tracks and 2 feet exterior thereto as provided by law, and shall defray the cost of excess construction occasioned by such use including the relocation and construction of closed plow pits at the west approach to the bridge in accordance with plans to be approved by the Mayor of the District of Columbia.
D.C. Code § 9-324