 Wadhurst Tunnel collapsed in 1862 and it was discovered by the SER that the same situation existed there too . Rectification cost £ 10 @,@ 231 . By 1877 , only one train was permitted in Bopeep Tunnel at a time . The tunnel was partly widened in 1934 – 35 . In November 1949 , serious defects were discovered in the tunnel . Single @-@ line working was put in place on 19 November , but the tunnel had to be closed completely a week later . The tunnel was partially relined with cast iron segments . It reopened to traffic on 5 June 1950 . Mountfield Tunnel was underpinned in 1938 – 39 , remaining open with single @-@ line working in operation . It partially collapsed on 17 November 1974 , resulting in single @-@ line working until 31 January 1975 . The line was then closed until 17 March whilst the track was singled through the tunnel . 
