the Harlem River tunnel; each measured long, with an interior diameter of , and were connected by a vertical cast iron diaphragm. The two tubes were surrounded by a layer of concrete measuring at least thick, while the roof was covered by a layer of concrete thick. An order issued by the United States War Department required that the top of the subway tunnel be at least below the tide level of the river. As a result, the tubes sloped downward at a 3 percent grade on either side of the river; these were the steepest sections of track built in Contract 1. Since the river bed contained clay, silt, and irregular rock, it could not be excavated using a conventional shield. Instead, the contractor suggested building a submerged rectangular cofferdam extending from the shore to the middle of the river, then excavating the riverbed and constructing the tunnel one half at a time. The Chief Engineer of the Rapid Transit Commission agreed to permit this method of tunnel construction, and work on the Harlem River tunnel began from the west side of the river in June 1901. The Mott Avenue station itself was built using cut-and-cover construction, since the poor composition of the rock in the area made it difficult for contractors to excavate the station using rock tunneling. The roof of the station was constructed as a massive concrete arch. The connection between the IRT White Plains Road Line and the IRT Lenox Avenue Line, including the 149th Street station, opened on July 10, 1905. Trains from the newly opened IRT subway ran via the line. Initially, the station was served by East Side express trains, which had their southern terminus at South Ferry and had their northern terminus at 145th Street or West Farms (180th Street). In 1906, West Farms express trains began operating through to Atlantic Avenue in Brooklyn. To address overcrowding, in 1909, the New York Public Service Commission proposed lengthening platforms at stations along the original IRT subway. As part of a modification to the IRT's construction contracts, made on January 18, 1910, the company was to lengthen station platforms to accommodate ten-car express and six-car local trains. In addition to $1.5 million (equivalent to $ million in ) spent on platform lengthening, $500,000 () was spent on building additional entrances and exits. It was anticipated that these improvements would increase capacity by 25