Great Western and Great Central Joint Railway

The Great Western and Great Central Joint Railway (GW&GCJR) was a joint venture supported by the Great Western Railway (GWR) and Great Central Railway (GCR) and run by the Great Western and Great Central Joint Committee. The original arrangement was agreed between the two companies in September 1898. The line and the working arrangement were authorised by the Great Western and Great Central Railway Companies Act of 1 August 1899 and opened in 1906. It was also known as the Alternative Route.

Early history
In 1898 the GCR's London Extension was in the final stages of construction, ready for its opening in March 1899. The London Extension had been designed to meet up with the Metropolitan Railway (MetR) tracks at Quainton Road north west of Aylesbury. Since the inception of the project relations between the Manchester, Sheffield and Lincolnshire Railway Company (the GCR's original name) and MetR had deteriorated markedly. This was one of the prime motivations on behalf of the GCR to bring the GW&GCJR into existence. When the route was completed it provided an alternative route for inter-city expresses from its wholly owned and operated London Extension from north of its junction with the MetR to its tracks at Northolt Junction on the new GCR line to Neasden Junction authorised by the Great Central Act of 12 August 1898. By the time of its opening the new pair of MetR double-tracks, from Neasden to the approach to Marylebone station had been ceded to the GCR in 1903. Thus the GCR was no longer at the mercy of the MetR. Ironically by that time reasonable relations had been established with the MetR.

The GW&GCJR included major upgrading of the GWR line between High Wycombe and Princes Risborough, converting it from single to double track. There is a two-mile section between Saunderton and Princes Risborough stations where the up and down lines follow different routes that run roughly parallel but up to 200 metres apart. The down (northbound) line follows the course of the original GWR single-track line, apart from a short section that was realigned slightly to the west in order to avoid the steeply-graded Saunderton Summit, whereas the up (southbound) line takes a slightly straighter and flatter route which passes through the short Saunderton Tunnel.

A junction was formed from the GCR tracks at Greatmoor a little to the east of Grendon Underwood and the link to Princes Risborough completed. At the southern end, another new line was built from High Wycombe via Beaconsfield, Gerrards Cross, Denham and Ruislip to Northolt Junction where the line divided: the northern path was the GCR's link to Neasden, and southern path led on to the GWR's mainline via Greenford.

Later History
Whilst the Great Central Railway was a constituent company of the London and North Eastern Railway, the title of the joint railway did not change at the 1923 Grouping, and in the Transport Act 1947 it was listed as one of the bodies whose undertakings are to be transferred to the British Transport Commission on 1 January 1948, thus becoming part of British Railways.

Presently, major sections of the route form the southern part of Chiltern Railways main line from Birmingham Snow Hill to London's Marylebone station via Bicester, specifically from Princes Risborough to Neasden Junction.