 Watkin did , however , obtain consent to extend the MR to Harrow , roughly 12 miles ( 19 km ) northwest of London , an extension which opened in 1880 . The Harrow line was further extended to Rickmansworth in 1887 . Rickmansworth at this time was a small town with a population of only 1 @,@ 800 ; to generate passenger traffic for the new station , a horse bus service between Chesham and Rickmansworth opened on 1 September 1887 . 
