King William Street was the original but short-lived northern terminus of the City and South London Railway (C&SLR), the first successful deep-level underground railway in London and one of the component parts of the London Underground's Northern line. It was located in the City of London, on King William Street, just south of the present Monument station. When King William Street was in operation the next station to the south was Borough and the southern terminus of the line was Stockwell.