The Portsmouth line is a secondary main line originally built by the London, Brighton and South Coast Railway and the London and South Western Railway between 1847 and 1868. It leaves the South London Line (Atlantic Lines) at , with connections to the Victoria branch of the Brighton Main Line at , and continues via , and to join the Mid-Sussex LineAlso known as the "Arun Valley Line", although that name is sometimes used only for the section from Horsham to Arundel, the actual Arun valley. at .
