There are lots of elements to the Thameslink Programme, but at its core is a proposal to untangle the lines outside London Bridge to get the maximum possible number of trains through as efficiently as possible.
As an introduction, here's how things currently work (click to enlarge):
[NB pedants: this is a simplified schematic showing the principle of the thing, not a track map, though it doubles as one in places]
The worst problems are highlighted in red:
- Thameslink trains through London Bridge have to cross in front of Southeastern services terminating at Blackfriars. There's also a short section of single track due to cost cutting before the Thameslink service was introduced.
- Borough Market Viaduct. A huge number of different services have to share this double track bottleneck, as well as the London Bridge platforms that serve it.
- To get onto the route towards Borough Market Viaduct, Thameslink trains need to make a flat crossing move.
- It's fairly common to have a pair of fast tracks between the two tracks used by stopping services, but normally there's a flyover provided near the terminus to switch them out. At London Bridge there is no flyover, so trains just cross the southbound slow line.
- Tanners Hill flydown. This link was built relatively recently to allow the bridge taking trains to Victoria* to double as a flyover to get trains onto the fast lines to London Bridge without crossing the slow lines. Currently only single track.
Here's how they intend to fix it:
- Rebuild Blackfriars to have the through tracks to Farringdon on the same side as London Bridge. The knock-on effect is that services from the Wimbledon/Sutton loop will now be on the wrong side to run through, so will terminate at Blackfriars, and services from southeast of Elephant & Castle will run through instead.
- Build a second pair of tracks over Borough Market. London Bridge will also be rebuilt with extra through platforms.
- The Bermondsey Diveunder. This is a massive piece of engineering due to the number of viaducts that need to be reworked. Its basic purpose is to get Thameslink trains to/from the Brighton direction over the top of the Southeastern tracks to/from Charing Cross.
- The southbound slow line is also routed under the dive under, and no longer crossed by other services.
- Double tracking the Tanners Hill Flydown. There aren't going to be many Thameslink trains going this way, but it's still part of the project.
[* The "temporary" replacement bridge built after the 1957 Lewisham Rail Crash, in fact]
[Thanks to Paul Scott for forwarding me the documents used to draw this]