 Supervision of the construction was lax , which enabled the contractors to skimp on the lining of the tunnels . This manifested itself in March 1855 when part of the brickwork of Mountfield Tunnel collapsed . An inspection of Grove Hill , Strawberry Hill and Wells tunnels revealed that they too had been constructed with too few layers of bricks . Grove Hill Tunnel had been built with just a single ring of bricks and no filling above the crown of the brickwork . The SER took the contractors to court and were awarded £ 3 @,@ 500 in damages . However , rectifying the situation cost the company £ 4 @,@ 700 . Although the contractors had charged for six rings of bricks , they had only used four . Due to the cost of reboring the tunnels , this had to be rectified by the addition of a further two rings of brickwork , reducing the width of the tunnels by 18 inches ( 460 mm ) . The result of this was that the loading gauge on the line was restricted , and special rolling stock had to be built , later becoming known as Restriction 0 rolling stock . This problem would affect the line until 1986 . 
