The Southsea Railway was a short railway branch line. It was built to give easier access from the jointly operated main line railway approaching Portsmouth to the Clarence Pier from which Isle of Wight ferries sailed. In 1879 the extension of the main line railway to Portsmouth Harbour station, where direct transfer from train to steamer was possible, eliminated most of the steamer business at Clarence Pier. Undeterred, promoters interested in developing Southsea projected the Southsea Railway, connecting a new Fratton station on the main line, with Southsea. The line was opened on 1 July 1885. Its independent promoters believed that it could be a main line terminus for London trains, and they constructed the line lavishly in consequence.

The London, Brighton and South Coast Railway was a railway company in the United Kingdom from 1846 to 1922. Its territory formed a rough triangle, with London at its apex, practically the whole coastline of Sussex as its base, and a large part of Surrey. It was bounded on its western side by the London and South Western Railway (L&SWR), which provided an alternative route to Portsmouth. On its eastern side the LB&SCR was bounded by the South Eastern Railway (SER)—later one component of the South Eastern and Chatham Railway (SE&CR)—which provided an alternative route to Bexhill, St Leonards-on-Sea, and Hastings. The LB&SCR had the most direct routes from London to the south coast seaside resorts of Brighton, Eastbourne, Worthing, Littlehampton and Bognor Regis, and to the ports of Newhaven and Shoreham-by-Sea. It served the inland towns and cities of Chichester, Horsham, East Grinstead and Lewes, and jointly served Croydon, Tunbridge Wells, Dorking and Guildford. At the London end was a complicated suburban and outer-suburban network of lines emanating from London Bridge and Victoria, and shared interests in two cross-London lines.