Thameslink is a mainline railway service which runs from the very south of England to Bedford via central London, stopping at Gatwick and Luton airports on the way.

The system began its life in 1989 and at the time of writing serves about 50 stations, although there are plans to extend services across more national rail lines in the future. However, the most important parts of the line still remain the Widened Lines and the Snow Hill tunnel, which allows it to run straight through London unhindered.

Early History

In 1863, the world's first underground railway, the Metropolitan Railway (MetR), was opened between Paddington Bishop's Road and Farringdon Street. Work soon began to link the MetR to the Great Northern Railway (GNR) lines at King's Cross station and construction was completed by the end of the year with a single track running from either side of the mainline terminus down onto the Metropolitan tracks, allowing GNR trains to terminate at Farringdon Street.

A year earlier in 1862, the London, Chatham & Dover Railway (LCDR) opened a new section of railway between Herne Hill and Blackfriars Bridge, the latter station lying just south of the River Thames. The first section was completed that year between Herne Hill and Elephant and Castle and it reached the south bank in 1864. The railway was soon extended northwards, with the line passing across the river to Ludgate Hill station, and was eventually linked to the Metropolitan line at Farringdon Street via Snow Hill tunnel in 1865.

At the same time, the MetR was extended to Moorgate station. Due to the vast increase in traffic during the line's first year, the plans for this extension included the construction of an extra pair of tracks between King's Cross and Moorgate for use by the mainline trains. These tracks were known as the Widened Lines and the first section between Moorgate and Farringdon was completed in 1866 - the whole project being completed in 1868. The new lines ran to the south of the MetR tracks, but then had to pass under the MetR lines and then into a tunnel to reach the overground lines at King's Cross.

Four years earlier, the Midland Railway had been given permission to construct a line from just outside their terminus at St Pancras, running under both Regent's Canal and the terminus building to reach the Widened Lines at King's Cross. The system of tunnels was eventually completed in 1871, when a chord¹ was built linking the Snow Hill tunnel directly to the line running towards Moorgate, allowing LCDR trains to run from the south into Moorgate. This tunnel section was destroyed during building works in the 1950s.

Snow Hill and Blackfriars

Despite running all of their trains out of Moorgate station towards the south via the Snow Hill tunnel, the LCDR was committed by an Act of Parliament in 1871 to build a new section of line. A 267 metre (292 yards) branch was to leave the course of the line just north of Ludgate Hill and run to a new terminus at Holborn Viaduct. The project was completed in 1874, and at the same time the LCDR opened a low-level station underneath the new terminus. This station was known as Snow Hill until 1912, when it was closed and renamed Holborn Viaduct Low Level to avoid confusion when trains were diverted into the closed station by mistake.

In 1886, a new station called St Paul's was opened on the north side of the Thames, with the line crossing the river parallel to the tracks into Blackfriars Bridge station. The latter station was then closed, with St Paul's station being linked to the Snow Hill tunnel and to the nearby Charing Cross station. St Paul's was renamed Blackfriars in 1937 and still serves the modern Thameslink route. However, the nearby Ludgate Hill station to the north slowly became disused and closed in 1923 when the local LCDR branch trains from Wimbledon stopped running.

The Fall and Rise of the Snow Hill Tunnel

Goods depots at various points along the Widened Lines continued to operate well into the 20th Century, providing a great deal of use for the tunnels. But local passenger services began to falter, leading to the closure of chord between Moorgate and Snow Hill in 1916 and passenger services from the north continued to pour into Moorgate via King's Cross and the goods link between the GNR in the north and the LCDR to the south remained open.

However, this situation could not last forever. The last GNR trains served Moorgate in 1977, the service having been made redundant when the Northern City Line, which had previously been a suburban line between Moorgate and Drayton Park, was extended to join the national railway lines at Finsbury Park. Freight services through the Snow Hill tunnel were withdrawn in 1969, with the track through the tunnel being lifted in 1971 and services from the south terminated at Holborn Viaduct.

The tunnel remained closed until 1988, when it was reopened as part of the Thameslink network. The line into Holborn Viaduct was closed and the route from Blackfriars was diverted down a steep incline, bypassing Ludgate Hill, into a new station beneath Holborn Viaduct station just south of the old Snow Hill station. This new station was originally named St Paul's, but was renamed City Thameslink for fear of confusion.

Structure

In central London, the Thameslink service runs from the overground lines at Blackfriars onto the new section of line towards City Thameslink. Trains then pass along the original route via Snow Hill tunnel and the Widened Lines and then north towards Bedford, though since 2007 they have stopped at a low level station underneath St Pancras instead of calling at King's Cross Thameslink. Trains can also run from the Moorgate terminus and head through St Pancras towards Bedford, although this route is poorly served.

South of London, trains have a choice of two routes. The first passes through London Bridge station and then runs all the way to Brighton via East Croydon and Gatwick Airport. The second is the LCDR line via Elephant and Castle, allowing trains to run on towards Streatham. When Thameslink was first opened, trains would pass from Streatham to Sutton via West Croydon and then carry on to Guildford, but this line became superfluous due to its closeness to the Brighton branch and the presence of other commuter lines to the Guildford area. The line now runs from Streatham to Sutton via Mitcham Junction, and then loops back towards the north again. There is a link between Tulse Hill on the Streatham line and East Croydon on the Brighton line, but this is rarely used. North of London, the trains use the Midland Main Line tracks to reach Luton and Bedford.

Due to differences in the main lines either side of London, trains are powered by overhead lines between Bedford and Farringdon, but then switch onto an electric third rail system towards the south. Apart from this unusual system, the trains are effectively the same as most 1980s-built trains, and each can carry about 300 passengers. The Thameslink stations outside central London usually have no entrance barriers unless they are shared by other services, and have the same appearance as most mainline stations.

Interchanges and Connections

On the other hand, the Thameslink platforms at Moorgate, Farringdon and Barbican are part of the tube stations there, which all serve the Metropolitan, Circle and Hammersmith & City lines, while Moorgate also provides a link to the Northern City Line towards Finsbury Park. Meanwhile, the station at Kentish Town has an entrance from the Underground ticket hall.

Wimbledon, London Bridge and Blackfriars mainline stations are all connected directly to their relevant Underground stations, serving the District, Northern and Circle and District lines respectively, but the tube stations at West Hampstead² and Elephant & Castle³ lie about ten minutes away on foot from the Thameslink stops. Other useful connections are to the Croydon Tramlink at Wimbledon, Mitcham Junction and East Croydon, to both Luton and Gatwick airports and to alternative national rail services at many stations.

The Future

There are plans to use the new tunnels being built for the Channel Tunnel Rail Link to connect the line at St Pancras to the Great Northern lines out of King's Cross. This would allow trains from King's Lynn, Cambridge and Peterborough to run through to Blackfriars and beyond, thus taking the pressure off the terminus at King's Cross. This scheme was first developed in 1991 as Thameslink 2000, but is now often referred to as the Thameslink Project.

Due to the projected increase in through-trains under the Project, there are plans to expand the Thameslink stations in central London. The Thameslink branch to Barbican and Moorgate will be closed to allow extension of the platforms at Farringdon, while London Bridge and Blackfriars will be improved. There are also plans to improve running times, with a dive -under⁴ at London Bridge to give Thameslink trains to Brighton an unimpeded route. These plans also include a controversial scheme to rebuild the historic Borough Market leaving room for the construction of a viaduct to allow Thameslink trains to pass over the South Eastern lines between Charing Cross and London Bridge.