 Except for its two stations — one at each end of the tunnel — the line ran entirely underground . The line was built in less than three months , and on its completion in 1936 was one of the shortest independent railway lines in the country . It was open for only three years before being converted to a major air @-@ raid shelter during World War II . After the war 's end , it was not included in the 1948 nationalisation of British railways but remained in private hands . 
