The station was rebuilt again with its current Portland stone entrance building by the Southern Railway (SR, the post grouping successor to the L&SWR) in the late 1920s as part of the SR's construction of the line to Sutton. Parliamentary approval for this line had been obtained by the Wimbledon and Sutton Railway (W&SR) in 1910 but work had been delayed by World War I. From the W&SR's inception, the DR was a shareholder of the company and had rights to run trains over the line when built. In the 1920s, the London Electric Railway (LER, precursor of London Underground) planned, through its ownership of the DR, to use part of the route for an extension of the City and South London Railway (C&SLR, now the Northern line) to Sutton. The SR objected and an agreement was reached that enabled the C&SLR to extend as far as Morden in exchange for the LER giving up its rights over the W&SR route. The SR subsequently built the line, one of the last to be built in the London area. It opened on 7 July 1929 to South Merton and to Sutton on 5 January 1930.