 The MR considered the Manning Wardle locomotives unreliable and from early 1903 they were replaced by a pair of Metropolitan Railway D Class engines; they were sold in 1911.The heavy D Class locomotives damaged the track, and in 1910 the track between Quainton Road and Brill was relaid to MR standards, using track removed from the inner London MR route but considered adequate for a rural branch line.Following this upgrading, the speed limit was increased to .