Apart from London Underground's network, the only other major railway tunnels under the centre of London are to be found on today's Thameslink line which connects London's northern and southern suburbs by passing over the tracks and tunnels of the so-called Widened Lines.
These lines have a history which stretches back nearly as far as the history of London's underground railways themselves as, for part of their journey, they parallel the Metropolitan and Circle line tracks between King's Cross and Moorgate.
The first section of this route opened shortly after the Metropolitan Railway started their services from Paddington to Farringdon Street in January 1863. At the end of that year the Great Northern company completed single line connections from each side of their King's Cross station to join the Metropolitan and started passenger services on 1st October through to Farringdon Street.
When the Metropolitan extended their services through to Moorgate Street (now Moorgate) in December 1865, a connection was also provided at West Street Junction at Farringdon to the London Chatham and Dover railway's new City Extension which had reached Holborn Viaduct (the future Low Level station). This was officially opened on 1st January 1866 and passenger services were operated by the LCDR from that date, while GNR goods trains used the link to reach South London from April the same year.
Initially the services all used the pair of mixed gauge lines built by the Metropolitan, but from the early days it was apparent that the volume of traffic would require additional tracks. Accordingly powers were obtained in 1864 to construct an additional pair of lines - the Widened Lines - from King's Cross to West Street Junction Farringdon. Authority for the additional tracks between there and Moorgate was included in the original application for the Moorgate extension so that, in fact, the first part of the Widened Lines to be completed was the section from Farrigdon and Moorgate which was opened to traffic in the first half on 1866.
The section from Farringdon to King's Cross took longer to construct, requiring a bored tunnel (rather than cut and cover) at Clarkenwell, the construction of the dive-under at Ray Street to the west of Farringdon (to enable the tracks to cross those of the Metropolitan) and major revisions to the layout at King's Cross which required the whole line to be closed down for a number of weeks. Eventually the line was re-opened and passenger traffic restarted in March 1868.
Meanwhile the Midland railway had also obtained powers to build its own connection to the Widened Lines in 1864 and to run a service direct to the City of London. They constructed a tunnel which left their mainline just before their St. Pancras terminus, descending below the Regent's Canal and the terminus station itself to join the Metropolitan's Widened Lines to the west of the GNR connection. This connection was completed in July 1868 and passenger services from the Northern suburbs served by the MR commenced to Moorgate immediately.
The final part of the jigsaw was completed in 1871 when a connection was built from Snow Hill Junction to Smithfield to provide a triangular junction and enable LCDR trains to reach Moorgate Street.
All the services were operated by steam locomotives fitted with condensing gear to reduce the levels of exhaust gases in the tunnels.
As can be seen from the map, there were a significant number of small goods depots along the line enabling the various operating companies easier access to their customers within Central London. All operated in very confined spaces as the whole line was constructed below normal street level. All operated well into the 20th Century, the old GWR one at Smithfield surviving until the 1960s.
However passenger services had mixed fortunes. While the link between the GNR and LCDR afforded by the widened lines continued to enjoy significant goods traffic, passenger services declined and were withdrawn in 1916, at which time the eastwards connection from Snow Hill to Moorgate was also closed. GNR and and MR services from the north into Moorgate however continued to flourish into British Railways ownership with rush-hour services from both lines. Diesel traction replaced steam only towards the end of full services from both lines, passenger services from the old GNR lines being finally ended in 1977. (New electric services were introduced in 1976 from Finsbury Park into the old separate Moorgate station of the Great Northern and City Railway - the tube station which as the scene of horrific Moorgate tube disaster.)
The current electric Thameslink services which link towns on the old MR mainline with a number of towns to the South of London (including Brighton and Gatwick Airport) started in 1988 when the Ludgate Hill to West Street was also reinstated (it had been officially closed since 1971 but disused for a few years prior to that).