Croxley Rail Link

The Croxley Rail Link, or the Metropolitan Line Extension, is a proposed railway engineering project in the Watford and Three Rivers districts of Hertfordshire, England, that would have connected the London Overground and the London Underground's Metropolitan line at Watford Junction. If the link were to go ahead, the Metropolitan line's terminus at Watford Underground station would be closed and the line diverted and extended from Croxley to Watford Junction via a reopened section of closed line. The main proponent of the scheme has been Hertfordshire County Council but it failed to win the support of Transport for London (TfL) which owns the Watford branch. The engineering works would have consisted of the realignment of the disused Watford and Rickmansworth Railway's line between Croxley Green and Watford High Street, with the construction of a viaduct over the Grand Union Canal, River Gade and A412 road and two new stations before branching into the London Overground line near Watford High Street and continuing to Watford Junction.

The proposed route was featured in a Transport for London network map for 2016 and the Transport for London Indicative 2025 Transport Map on which Watford tube station does not appear. Were the link to be built, direct services between Watford Junction and Amersham would be possible but were not included in the business case for the scheme.

The project was approved by the government on 14 December 2011 and vegetation clearance started during 2013. The project received final government approval through an order under the Transport and Works Act on 24 July 2013 and was signed off and formally given the green light by the government on 17 March 2015 when it was confirmed that Transport for London would be responsible. Work on the extension stopped in 2016 due to anticipated cost overruns and an unresolved dispute over funding. London mayor Sadiq Khan announced in 2018 that there was no funding for the project.