Between 1899 and 1923, the SE&CR had a monopoly of railway services in Kent and to the main Channel ports for ferries to France and Belgium.

This merged the two companies' operations, although they remained legally separate, with receipts split 59% to SER and 41% LC&DR until the Grouping, to avoid the costs and risks of a formal merger.

With the development and implementation of electrification by the L&SWR, the LB&SCR, the "Tube" companies and tram operators in the early twentieth century, the SE&CR planned to start electrifying its lines.

[8][9] This very high voltage for rail track level systems was used in Britain only on the L&YR's 1200 V DC side-contact third-rail line from Manchester Victoria to Bury.

Grouping in 1923 led to the Southern Railway adopting the L&SWR's standard of 660 V DC third rail on the SE&CR's network.