 The new line 's infrastructure was designed by Henry Greenly , a leading figure in the design of narrow gauge railways . He had begun his career at the Metropolitan Railway ( now part of the London Underground ) , and had designed the route , buildings , locomotives and rolling stock for the Rhyl Miniature Railway and the nearby Romney , Hythe and Dymchurch Railway . As the new railway would not be carrying heavy loads and would be travelling only a short distance , it was built as a narrow gauge railway , with a track gauge of 2 ft . This allowed the new branch tunnel to be built to far smaller dimensions than the existing tunnel , at just 8 feet ( 2 @.@ 4 m ) high and 6 feet ( 1 @.@ 8 m ) wide . 
