What is... the Thameslink Programme?

The Thameslink Programme is a £5.5 billion project to triple the capacity of the Thameslink line, which runs north-south through central London via St Pancras, Farringdon, Blackfriars and London Bridge. If you have no earthly idea what Thameslink is, I suggest you start by reading my previous post, which explains the history and background of the line. In this post, I'll try and explain how the £5.5 billion will be spent.

Until 2009, the Thameslink core between Farringdon and Blackfriars could only cope with a maximum of 8 trains per hour (tph) in each direction. Furthermore, the platforms at King's Cross Thameslink, Farringdon and Blackfriars were only long enough to take 8-car trains, even though many of the stations further out could cope with 12-car trains. As such, the Thameslink trains are horrendously overcrowded.

Indeed, Thameslink was horrendously overcrowded within just a few short years of the line opening in 1988: passenger numbers quadrupled within the first year. So in November 1991, a major upgrade, then entitled "Thameslink 2000", was announced. Through numerous delays, first due to privatisation and then to complex planning inquiries, the programme was only approved in 2006 and funding was forthcoming the following year. It was quietly renamed "the Thameslink Programme" to hide the fact that it was running about 18 years late.

The plan had a simple aim: convert Thameslink into a proper RER-style cross-London line with 24 trains every hour, most of them 12 carriages long, running in each direction in the rush hour. This necessitated a variety of modifications:

• more destinations, both north and south, were needed to send trains to;
• longer platforms were required in the Thameslink core;
• the branch to Moorgate had to close;
• most importantly, London Bridge had to be rebuilt to give it the necessary capacity.

The need for additional destinations was simple: while the core could be upgraded to cope with 24tph, there wasn't any need for 24tph to Bedford or Brighton. To the north, Thameslink connected only to the Midland Main Line (MML) out of St Pancras, but that took it quite close to the East Coast Main Line (ECML) out of King's Cross. Peak-time services on the MML already amounted to 14tph, so a connection to the ECML would provide enough capacity to the north to run 24tph through the core.

As luck would have it, such a connection used to exist: the lines now used by Thameslink - the "City Widened Lines" - had not one but two historical connections to the ECML (as seen in this diagram), one of which had only closed in 1977. Thought was given to simply reinstating one of these connections, but ultimately it was decided against just putting back the old connection, for one simple reason: it would have to be a flat junction, and that would have constrained capacity too much to ensure that 24tph could be run reliably.

A flat junction on the railway is much like a right-turning lane on a dual carriageway: trains wanting to turn right must wait for a gap in trains coming the other direction to make the turn. Grade-separated junctions, usually called "flying junctions" on the railway, is akin to a proper motorway junction: the conflicting movements are replaced with a bridge or a tunnel.

The principal downside to having a flying junction is space: even in the diagram above it can be seen that a flying junction takes up more land, and land in central London is nothing if not scarce and expensive. And while the old connections to the ECML could have been used, there wasn't the space around them to make them into flying junctions.

Instead, a plan for something much grander came about: St Pancras would be comprehensively rebuilt for the Channel Tunnel Rail Link so that Eurostar trains could use the station. In the process, two new underground platforms would be provided for Thameslink services under St Pancras, replacing the inconveniently-sited and impossible-to-enlarge King's Cross Thameslink station. Just north of the new station, a flying junction with a connection to the ECML would be built - not an easy task given the incredible honeycomb of tunnels in the area, with six London Underground lines serving King's Cross-St Pancras.

In spite of the fact that the Thameslink programme still hadn't been finally approved, the construction at St Pancras was given the go-ahead as part of the Eurostar upgrade, with Thameslink services split in two for nine months while the new station was constructed. Thameslink duly moved into its new home at St Pancras in December 2007, with King's Cross Thameslink closing at the same time. The tunnels for the flying junction were bored, but left without track for the time being: they won't be needed until 2018.

At Farringdon, the platforms were only long enough to take 8 carriages: extending them to the north was impossible, due to the severe gradient. Extending them to the south, however, would entail removing the flat junction to Moorgate. From the point of view of improving Thameslink services this was actually a benefit: without the trains to Moorgate, there would be more room for trains to Blackfriars and beyond. Passengers for Moorgate would still be able to change at Farringdon and get the Metropolitan line to their destination.

At Blackfriars, there were five platforms, all only long enough for 8 carriages; but three were bay platforms, only accessible to trains terminating from the south. Unfortunately the bay platforms were to the east of the through platforms, meaning any terminating trains had to cross the path of Thameslink services (another flat junction). Ideally the bay platforms would be to the west of the through platforms; to achieve that, the plan involved extending Blackfriars considerably to the south, to permit the through lines to be slued across.

In fact, extending Blackfriars to the south came up across a large obstacle: the River Thames. The platforms already encroached a little onto the bridge carrying the railway over the Thames; the Thameslink Programme called for them to be extended all the way across the river. Indeed, the plans included a new entrance to Blackfriars station on the South Bank, with the station having four platforms - two bays on the west side, and two through platforms on the east side.

Phase 1 of the Thameslink Programme, begun in 2009 and finished in time for the Olympics in 2012, involved shutting the Moorgate branch and the bay platforms at Blackfriars and diverting those services through the core (increasing peak services to 15tph) to facilitate the construction works at Farringdon and Blackfriars. In addition, the line was completely resignalled to permit trains to run much closer together. The works were not without disruption, requiring nearly three years of weekend closures through the Thameslink core.

But the disruption there pales in comparison to that caused by Phase 2: begun in 2013 and due for completion in 2018, Phase 2 is focussed almost entirely on London Bridge. Currently, Thameslink trains currently have to share tracks with trains to and from Charing Cross, and then cross over to the line towards East Croydon in a double flat junction. Prior to the works, the layout looked like this:

London Bridge track layout in 2009

By 2018, Thameslink will have its own dedicated platforms and tracks through London Bridge, and the layout will look like this:

London Bridge track layout in 2018 (provisional)

(To see clearly exactly what's changing, try opening both and flicking back and forth between the two.)

As can be seen, Thameslink had pretty much no track of its own through London Bridge. In a hugely complex plan, two extra tracks will be constructed between Metropolitan Junction and London Bridge for Charing Cross trains, with Thameslink taking over the existing two tracks. The lines run over the top of Borough Market, and finding space for the extra viaducts has been an absolute nightmare: the formation for the new tracks was put in place in 2011, involving one of the most complicated bridge slides ever done (see these pictures).

A couple of miles out at South Bermondsey, a flyover will be constructed to permit Thameslink trains to jump over the Charing Cross trains without interrupting them. This involves completely reconfiguring which track is which, and will not be an easy task.

Crucially, though, there aren't enough through platforms at London Bridge for those lines on their own to actually be any use: there were 6 through platforms and 9 bays for services terminating from the south. That will be turned into 9 through platforms and just 6 bays, through a massive gradual reconstruction; only three platforms will be closed at any one time, and a full peak service will continue to run throughout (though some trains will not call at London Bridge during the works).

Over the course of 2013 and 2014, the nine old bay platforms will be shut and the six newly-repositioned bay platforms opened in their place. From January 2015 to August 2016, no trains to and from Charing Cross will call at London Bridge, to permit the new through platforms 6-9 to be constructed. From then until early 2018, no trains to and from Cannon Street will call at London Bridge, permitting the rest of the through platforms to be reconstructed. Notably, only five of the six existing through platforms will remain: platform 1 will be removed, permitting all the platforms to be straightened somewhat (they are currently quite curved).

Perhaps the most annoying thing during the work will be the diversion of Thameslink services. As discussed previously, there isn't enough capacity for Thameslink services to serve London Bridge in the peak, and so instead they run via Elephant and Castle. But for the three years from 2015-2018 when the through platforms are being reconstructed, no Thameslink services will run via London Bridge at all, even in the off-peak.

But once the rebuilding is complete, Thameslink trains will be able to run through London Bridge all day, with up to 18tph serving London Bridge (with the rest going via Elephant and Castle). Beyond London Bridge most trains will continue to head south towards East Croydon and on to various destinations off the Brighton Main Line, but the capability will be there for trains to head towards Kent as well (with a flying junction at Bermondsey). Once London Bridge is completed, the connection to the ECML will finally be opened for use and Thameslink trains will serve a wide variety of destinations.

You might notice I'm being slightly coy about exactly where it's going: that's because it hasn't really been decided yet. While the infrastructure upgrades for the core are clearly defined, the end network is not (unlike Crossrail). Describing the probable destinations, and the politics involved therein, would be another article in itself; I'll leave it to the good people at London Reconnections, who have a fascinating article on the possible destinations of Thameslink post-2018, which I highly recommend.

In the meantime, though, we are right in the middle of a hugely exciting but also horrendously disruptive upgrade of London Bridge, that will unlock the potential of Thameslink in a way that this country has never seen before. The London railway network post-2019, once both Thameslink and Crossrail are operational, will be quite a different place, and I look forward to it all coming to fruition. To keep track of what's going on, you can check the Thameslink programme website.

Both Crossrail and Thameslink will achieve their 24tph through brand-new fleets of trains running with Automatic Train Operation (ATO). What is ATO, I hear you cry? Head on over to the next post...

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