The Chiltern Main Line is an inter-city and commuter railway in the United Kingdom, part of the British railway system. It links London and Birmingham on a 106 mile route which runs via North West London, Buckinghamshire, Oxfordshire and Warwickshire.
The line complements the West Coast Main Line by providing an additional inter-city link between England's two largest cities, as well providing commuter services between London and Buckinghamshire and beyond.
Passenger services on the route are currently operated by Chiltern Railways. Some services between Birmingham Snow Hill and Leamington Spa are operated by London Midland, whilst CrossCountry run services over the line between Birmingham New Street and Coventry to Banbury as part of their services from Scotland and the north of England to the south coast of England.
For many years, trains from Birmingham to London went the long way round via Oxford and then along the Great Western Main Line to London Paddington. In an attempt to compete with the LNWR's London-Birmingham route, the GWR took advantage of an existing partnership with the Great Central Railway (GCR) to build a new, direct route referred to as the Great Western and Great Central Joint Railway (the two companies were already working together over a link between Woodford Halse station and Banbury station).
- a direct line, referred to as the New North Main Line, towards London Paddington via Greenford and Old Oak Common, built by the GWR. This line also included a triangular junction at Greenford, providing access to the 1838 Great Western Main Line via Castle Bar Park.
- a line via South Harrow and Wembley to meet the existing GCR route at Neasden Junction, near Neasden station on the Jubilee Line. This is the line used by almost all Chiltern services.
The new route runs northwest via Gerrards Cross to High Wycombe, where the line met the existing route of the Wycombe Railway's single-track branch from Maidenhead. North of High Wycombe station the existing route was reused, with significant upgrading of the formation, and a new route chosen for the up line north of Saunderton (the existing route, taken by the down line, has a gradient of 1 in 88, too steep for the heavy coal trains run by the GCR). Upon reaching Princes Risborough, new construction was started again, running northwest to a point near the hamlet of Dorton, where the line curved to the north to meet the GCR's main line at Grendon Underwood Junction in Buckinghamshire. The line was completed in 1906.
Four years later, the GWR constructed a second line, starting at Ashendon Junction in Buckinghamshire, near Dorton, and running northwest via Bicester to meet the existing route from Oxford at Aynho Junction. This truncated the original Joint Line to Ashendon Junction, the section northwards of there being sold to the GCR after the completion of the Bicester cut-off.
The new shortcut provided both a faster route between London Paddington and Birmingham Snow Hill stations for the GWR, and a diversionary route for the GCR that would allow it to avoid the route of the Metropolitan Railway. During the heyday of the route, many prestigious trains ran from Paddington to the northwest of England via the Joint Line, reaching Wolverhampton, Chester, Liverpool and Birkenhead. Various through services from Marylebone to the GCR network also ran via the Joint Line.
In the 1960s, when the rival West Coast Main Line (WCML) was electrified, express services from London to Birmingham on this route were discontinued as part of the Beeching Axe; beforehand, it was even more heavily used by many long trains running from Liverpool and Birkenhead, as the WCML was restricted in capacity due to the electrification works. All local trains on the route were diverted to Marylebone in 1963, and Greenford station on the New North route between Old Oak Common and Northolt Junction was run down and eventually closed. The route was downgraded to secondary status in 1967, and subsequently single-tracked between Princes Risborough and Aynho Junction, which remained a flying junction. Snow Hill station in Birmingham was also closed, along with the line to Wolverhampton.
The route was eventually considered for partial closure in the 1980s, with all services returning to Paddington via the New North route, and Marylebone station and all lines leading to it being closed - services to and from Aylesbury would have run via Princes Risborough. Marylebone was formally reprieved in 1986, however, and the closure proposals were rescinded.
Services were expanded somewhat in the late 1980s, when Snow Hill station was re-opened, although they still ran from Marylebone rather than Paddington. In the early 1990s, the New North route between Old Oak Common and Northolt Junction was singled between Old Oak Common and Park Royal and also between Greenford and Northolt Junction. The Total Route Modernisation performed by BR in the early 1990s removed most of the vestigial traces of main line heritage from the route, downgrading it purely to a commuter line with a minimum of available infrastructure; until that point, High Wycombe station alone had retained almost all of its original track layout, the other major stations on the line having already been downgraded. In 1992, the old signal box at Aynho Junction was closed and replaced with modern signalling controlled from Banbury South signal box; the structure stood until 2002, when it was demolished. As part of these renovations, BR also installed the advanced Automatic Train Protection (ATP) system mainly as a trial with a view to rolling it out nationwide. However, privatisation intervened, and the Great Western Main Line was the only other line to be equipped with ATP.
Upon rail privatisation in the 1990s Chiltern Railways took over the route, and in 1998, the line between Princes Risborough and Bicester North was redoubled by the company. This included the total reconstruction of Haddenham and Thame Parkway station at platform level, with two side platforms instead of the single platform constructed in 1987. In 2002, after Chiltern won their 20-year franchise, the line between Bicester North and Aynho Junction was also redoubled.
Part of the old Great Western route from Birmingham Snow Hill to Wolverhampton is now used by the Midland Metro light rail system.
The line from Northolt Junction to Paddington alone has not been improved, and only one Chiltern train a day from Princes Risborough, and back, uses it, and only during the week (except on Wednesdays). Freight trains carrying refuse from London use the line, however, and it has been used as a diversion when work is taking place on the line to Marylebone, or when the normal line into Paddington is closed. HSTs are often sent around the Greenford loop via Ealing Broadway and Park Royal to turn them around for operational reasons.
As of September 2006, Chiltern has completed their Evergreen 2 upgrade project. The project, which was carried out by Carillion, realigned the track through Beaconsfield to increase non-stop speeds from 40mph to 75mph, installed additional signals between High Wycombe and Bicester North (as well as between Princes Risborough and Aylesbury), and added two new platforms to Marylebone. The new platforms are on the site of the old daytime carriage sidings, which were replaced with the new Wembley Light Maintenance Depot, just to the south and west of Wembley Stadium station. The new platforms and partial resignalling of the station throat now make it possible to run 20 trains per hour in and out of the station.
In January 2007, Chiltern Railways became the first rail operator in the UK to sell train tickets and fulfill these to customers' mobile phones. The txt2mobile fulfilment method is available for Chiltern's advance purchase E-Day products. A barcode ticket is sent to the traveler's mobile phone using the SMS format, and this ticket can be scanned on the train or at the gate line at Marylebone Station. The ticket is then verified against a database of valid tickets.
- A new station will be built in north Aylesbury, called Aylesbury Vale Parkway. The planned site is 2 miles north of Aylesbury station along the Calvert freight-only line, which would have upgraded signalling fitted. The service would be hourly, with up to 3 arrivals / departures per hour in the peak times. The station is expected to open in 2009.
- The restoration of the quadruple track between South Ruislip and West Ruislip, allowing trains to call at both stations without blocking the line. Triple track currently exists at West Ruislip, with the up platform loop still in situ, and at South Ruislip, with the Down Main through line also in situ. This would involve the reconstruction of the down platform at West Ruislip, the reconstruction of the up platform at South Ruislip, and the demolition of West Ruislip signal box.
- Building of the West Hampstead Interchange to allow easy interchange with the Silverlink Metro, Jubilee line, Metropolitan Line and Thameslink line. This would give Chiltern Railways an interchange with the future Orbirail line.
- A new railway built between Oxford and Princes Risborough, this would then give Oxford an alternative to the Oxford-Paddington route. The Oxford to Banbury spur would then be handed over to the Chiltern mainline to create a diversionary loop from Princes Risborough to Banbury via Oxford. This option requires an expensive crossing of the M40 motorway.
Plans that are at an early stage have proposed a Crossrail-type scheme to link Marylebone and Fenchurch Street station. This would give extra east west capacity through London and link two of the smaller rail franchises that could then be amalgamated.