Located in Hertfordshire, both Rickmansworth and Croxley stations relate to one of my favourite facts about the London Underground. Both stations are served by the Metropolitan line, but Rickmansworth also receives National Rail services from Chiltern Railways on the London to Aylesbury line. Rickmansworth and Croxley stations are located in zone 7 and feel a million miles away from zone 1 and central London.
Ricmansworth opened in September 1887 as a terminating station of the Metropolitan Railway as the company extended services from Pinner. Metropolitan Railway services extended to Chesham two years later in 1889. The Metropolitan Railway would eventually terminate in Brill, Verney Junction and Aylesbury, but services this far out of London gradually closed from 1935 to 1961, predominantly due to the electrification of the majority of the line. Electrified services began serving Rickmansworth in January 1925, and the final steam service to leave Rickmansworth was the Rickmansworth-Watford shuttle in September 1961.
The Rickmansworth-Watford shuttle was re-introduced in 1987, however, just one each way a day, the Rickmansworth to Watford service left early morning, and the Watford to Rickmansworth service left in the late evening. When S8 Stock trains were introduced, the shuttle was extended to Amersham to Watford, but still just one service each way.
The shuttle utilises the North Curve, a curve a track that links Rickmansworth and Croxley. Due to the infrequency of services, this track does not appear on the London Underground map. Throughout the rest of the day, if you want to travel between the two stations, you will have to change at Moor Park. The North Curve that connects Rickmansworth and Croxley goes under a tunnel, which is the only tunnel on the London Underground and point where services go underground, outside of the London.
Croxley station opened in November 1925 when the Metropolitan Railway extended to Watford. The station was originally named Croxley Green, however, the nearby Croxley Green station operated by London and North Western Railway led to confusion. In 1949 the Metropolitan Railway station was renamed to simply Croxley. The London and North Western Railway's Croxley Green was temporarily closed in 1996, but by 2003 it was decided that the station and the rest of the Watford-Rickmansworth line would be permanently closed. 
Major engineering works were planned to take place just north of Croxley station as part of the Metropolitan line extension. Trains would have diverted onto the Croxley Rail link, and new stations would have opened at Cassiobridge and Watford Vicarage Road, and continue to Watford High Street and Watford Junction. These works would have resulted in a closure at Watford station. Despite works commencing, it was announced in January 2017 that works had stopped due to a funding issue.
Why not board a train and travel in the only underground section of the London Underground outside of London? If you do not fancy getting up early, you can best spot the tunnel from London bound services between Rickmansworth and Moor Park.
