 D class locomotives , introduced by the MR to improve services on the former Tramway line , damaged the track , and in 1910 the line between Quainton Road and Brill was relaid to MR standards using old track removed from the inner London MR route , still considered adequate for light use on a rural branch line . Following this track upgrading , the speed limit was increased to 25 miles per hour ( 40 km / h ) . The MR was unhappy with the performance and safety record of the D Class locomotives , and sold them to other railways between 1916 and 1922 , replacing them with A class locomotives . 
