Making the Grade (Separation): The Bermondsey Diveunder


In the past we have resisted the temptation to write too much about the Bermondsey Diveunder as work had not really started in earnest and it was very hard to visualise what was planned. With demolition now well and truly underway it is time to rectify this and look at this absolutely vital part of the Thameslink Programme.


BDU annotated
Revised Legend

For the benefit of those unfamiliar with the area we start with an annotated picture taken fairly recently from the Network Rail helicopter looking roughly south east. The most easily identifiable landmark is The Den, home of Millwall Football Club. Just to the left of that and slightly further away in the top centre is the Combined Heat and Power plant where much of London’s rubbish gets burned. To the left of that is London Overground’s Silwood sidings. There appear to be a number of trains parked there but what is probably being seen is the concrete track base on which they are stabled.

Disappearing off the centre top is the the Brighton Main Line from London Bridge towards East Croydon. New Cross Gate is top centre. It cannot be made out, but the annotation of the bridge taking the London Overground over the Brighton Main Line is clearly marked. The line through South Bermondsey is to the right with South Bermondsey station present in the picture but extremely difficult to pick out at this scale.

The Cannon Street Lines are the leftmost tracks (marked in Southeastern purple) near the top of the picture. They separate at North Kent East Junction with route to the left going to Deptford and Greenwich and the route to the right going to New Cross. Like New Cross Gate, the location of the station can be identified by the annotation of the London Overground route.

The site of works on the diveunder itself can be seen alongside the Charing Cross (blue) line at the bottom of the picture. Not so obvious is the extent of it. It occupies roughly a third of the height of the picture.

Carto Metro is surprisingly up-to-date and it clearly shows the former and current track layout depicted on the photo. As always those who want a close up view can find a full set of photos by Unravelled in his Bermondsey Diveunder set.


We have said this all before but it is probably worth repeating. A major objective of the Thameslink Programme is to provide a service through the Thameslink central core to and from London Bridge. To do this two pieces of infrastructure are absolutely vital. One is the new viaduct at Borough Market, which provides for an additional two tracks at this otherwise critical pinchpoint. The other is a grade-separated crossing between New Cross/New Cross Gate and London Bridge station that enables trains from the South East to reach Charing Cross without conflicting with trains from the Brighton Line going to Blackfriars and beyond. This grade-separated structure has become known as the Bermondsey Diveunder.

It is of course also true that a lot of work needs to be done at London Bridge station as part of the Thameslink Programme, but the extent of the work involved here is partly due to a conscious decision to take advantage of the intended works and comprehensively redevelop the station at the same time. As the station, especially the notorious old platform 6, was becoming quite unfit for purpose, a rebuild of some kind would have probably happened whether the Thameslink Programme went ahead or not.

Although, in financial terms, the grade separation is a small part of the overall project, it is nevertheless a vital part of the grand plan. It is also probably the most disruptive element of the entire programme.

The problems at London Bridge at the start of the year are well known. Although the cause is generally accepted to be a combination of timetabling issues and and various other factors, especially signalling problems, the fundamental reason for the disruption and current restrictive service on the Brighton Main Line into London Bridge is the limited number of tracks now available between New Cross Gate and London Bridge. This in turn is caused by the need to take tracks out of service in order to build the necessary grade separation.

Making the grade (separation)

The scheme selected for the grade separation was certainly not the same as original proposed. Grade separation schemes are difficult to add to an existing railway and all the more so in urban areas where acquiring additional land can be fraught with problems. Particular issues in the Bermondsey area were that the railway was already on a high viaduct and the restricted amount of space available due to nearby industrial units. A factor that created many changes was the proposed reopening of the former railway between Surrey Quays and Queens Road Peckham – now a reality as part of London Overground. One major consequence of this was that this reopened railway determined the easternmost location at which Charing Cross bound trains from New Cross could start to descend to go underneath the Thameslink tracks.

Bermondsey DiveUnder

The classic Network Rail view of the Diveunder.

The reason for choosing the Charing Cross tracks as the ones that dived under was probably because it was the simplest option. There wouldn’t be any point work or junctions on a gradient – which is what you would have probably got if it were the Thameslink tracks that dived under. The Charing Cross tracks were also going to be much less complicated than those on the Brighton Line. They are just a simple pair of tracks going through New Cross and, in essence, in the future it will remain double track all the way to just short of London Bridge where each track will split into two tracks to serve an island platform. So the up line will split to serve platforms 8 and 9 at London Bridge and the down lines serving platforms 6 and 7 at London Bridge will converge just to the east of that station.

Existing and Proposed

Before and after looking towards the south-east – superficially not much difference

In simple terms, the scheme is one where the existing Charing Cross tracks dive under two tracks reallocated to the new dedicated Thameslink service. Most tracks would, in two dimensions, be in roughly the same location as before the scheme started. A slight realignment of the routes involved would enable the two critical routes (Thameslink and Charing Cross services) to switch over from one side to the other. It would be the vertical height that would be the significant change.

In the early days, when the scheme was proposed, much emphasis was made of the benefits of “reusing” a line to the former Bricklayers Arms Yard (BAY). In practice, as is so often the case with old disused structures, it actually made more sense to demolish what was there and rebuild it to modern standards whilst one had the chance. In any case it turned out that the rebuilt line to Bricklayers Arms was not really a vital component of the scheme in the final plan.

It’s getting there that is the hard bit

The considerable disadvantage of the plan to build the Bermondsey Diveunder was one could not dramatically alter track levels whilst they were still in use. Nor could one realistically build the massive concrete box which was the heart of the structure and continue to have trains running through a massive worksite. It was clear that the number of tracks available for trains was going to have to be considerably restricted whilst work was going on.

On the SouthEastern side the three tracks to and from Cannon Street have been temporarily reduced to two at the location of the future diveunder. There is now, as there will be in the future, just one up and one down Charing Cross track – not a problem provided things don’t go wrong.

There is more of a problem on the Brighton Line during construction as there is, for a short distance, just one up and one down line where there used to be two of each. At New Cross Gate and continuing all the way to Three Bridges there are two up and two down lines (or more) and fast and slow trains on these now have to share tracks just north of New Cross Gate station. The only small relief is that the 8tph London Overground service which uses the slow lines from the south has already diverted onto the East London Line before the two track section is reached.

Construction of the ramp

The first major sign of construction was the replacement of the descending brick viaduct of the former line that led to Bricklayers Arms freight depot with a modern concrete ramp. Although the original line to Bricklayers Arms was double track, the replacement ramp is only intended for a single track. Because this was a disused line, work commenced long before the main work was started and is already practically complete. However, despite being the first of four “diveunder” trackbeds to be completed, it will actually be the last of the four to be brought into operation.

Spur to Cannon St Lines

The ramp is practically complete as far as construction goes.

When trying to work out what is planned and where, the already constructed ramp is very useful because it gives us our first point of reference. This ramp will support the track that comes off the Charing Cross down line and proceeds to join the Cannon Street reversible line in the down direction in the New Cross area by crossing the Cannon Street up line on the flat. The picture below shows where the spur line will join the main route. At this location the track from the ramp (behind the photographer) will continue in the foreground to join the current right most track of the four shown. Although there will be a junction with a set of points here, the other route would not be in normal use as it branches off the Thameslink down line just prior to reaching the diveunder. Therefore any train using the ramp could expect a clear run and should not be halted on the ramp.

Spur joining main line

Where the spur ramp joins the main route. Looking eastwards towards New Cross.

One could reasonably ask what the point of a track joining the Charing Cross down line to the Cannon Street down line is. After all, a great deal of fuss was made about making all Greenwich services go to Cannon Street to avoid conflict yet here Network Rail seem to be unnecessarily introducing it. The answer can be found when considering those trains that will in future depart from Charing Cross to go to Lewisham platform 4 and onward to Blackheath. If they come up the ramp and continue to New Cross then they only have to cross the Cannon Street up line. However if they stay on the Charing Cross down line then they have to cross the Charing Cross up line to get on to the Tanners Hill flydown and then negotiate the complex junction just west of Lewisham station to get to platform 4 at Lewisham. This is particularly desirable when the train involved is a 12-car as a 12-car down train halted at the top of Tanners Hill flydown will foul the main line and effectively bring all Charing Cross services to a halt.

The diving Charing Cross Lines

Moving on to our second point of reference we now consider the Charing Cross up and down lines. The Charing Cross lines are the “fast” lines from Hither Green. The tracks from Tanners Hill flydown join these tracks just to the west of St Johns. As they approach the diveunder from the east, the 2D future location of the Charing Cross tracks is same as the southernmost two tracks before work started. At present, beyond New Cross towards London, the two remaining Charing Cross tracks are diverted to occupy adjacent lines. This means that the pair of tracks that will eventually use the diveunder are currently unused.

In construction terms, what has now happened is that the approach from New Cross to the diveunder used by the future Charing Cross tracks has had the top of the viaduct demolished. The structural base of the arches formerly supporting the viaduct have only been cut down to a suitable height for re-use. Because of the unusual properties of cement these bases are probably now even stronger than when they were built and there is no point in completely demolishing a sound structure that can be reused.

The photo below, taken by ngh, shows the stepping down of the base structures of the original foundations of the viaduct when looking towards New Cross.

ngh view of CX lines from ramp

The Charing Cross lines that will eventually drop down to go under the Thameslink ones. Looking South East with the Combined Heat and Power (CHP) plant in the background.

The following picture, taken by Unravelled from the other direction, also shows how the height of the walls of the former arches are gradually being reduced in preparation for rebuilding the Charing Cross Lines. Note the proximity of the previously described ramp. These will be side-by-side once they reach the diveunder itself.

Charing Cross Lines looking the other way

Charing Cross lines looking towards London Bridge from the path by the East London Line

This pair of Charing Cross tracks will emerge from the western end of the diveunder to line up with the former Charing Cross up line and the former Charing Cross up platform loop. These former up lines will then become a continuation of the two-track Charing Cross up and down lines.

Last but not least

We have now identified three of the four tracks that will be “downstairs” in the diveunder. The final one to the left (south) of the Charing Cross pair of tracks simply takes trains from London Bridge terminating platforms to the slow down line at New Cross Gate and utilises the diveunder to do this by going underneath the Thameslink tracks (which effectively become the fast lines to East Croydon). The trackbed for this line should be ready by summer 2016 in order to allow six months for track and signalling to be installed and tested. In January 2017 this should then become the first section of grade separated track to be brought into use and finally provide some much needed relief for the lines out of London Bridge low level platforms. By then there will at last be three tracks all the way from London Bridge to New Cross Gate whereas currently there are only two just north of New Cross Gate.

January 2017 with extra line in use

The Brighton Line tracks as they are planned to be at January 2017

The Thameslink Line from the south

Next we need to look at the Thameslink Line to and from New Cross Gate. These will be the centre two tracks at New Cross Gate (ignoring the easternmost track which is London Overground only). Now, as stated before, everything will be roughly on the same alignment as previously. Although the Thameslink Line will be on almost the same alignment as previous tracks (not called Thameslink then), it is going to have to approach the top of the diveunder from a slightly different angle to cross it obliquely and come out aligned to the new Thameslink tracks. These will replace the former lines 4 and 5 out of London Bridge – both lines having previously been down Charing Cross lines.

Bolina Bridge being demolished

Bolina Bridge being demolished

Because the Thameslink Line from New Cross Gate needs to approach the diveunder construction at a slightly different angle, the entire embankment needs to be shifted. Given the current state of knowledge about constructing embankments and that known at the time of construction, it makes sense to demolish and rebuild the entire embankment whilst one has the opportunity. Hardcore is being crushed and reused on site to avoid unnecessary transhipment of spoil. Demolition started at the beginning of June 2015 and has been remarkably rapid. This demolition has included the bridge over Bolina Road which will also be rebuilt. Bolina Road is currently closed but will be reinstated at the end of the works.

Recycling Viaduct

Recycling a viaduct for reuse

Once beyond the concrete structure housing the diveunder, these two Thameslink tracks will continue to a new island platform at London Bridge with the north face being platform 4 and serving the Thameslink down line, and the south face being platform 5 and serving the Thameslink up line.

Clearing Rotherhithe New Road

It was previously stated that the Charing Cross tracks approaching the diveunder from the south east could not start to dive until they had cleared the East London Line. To the west of the diveunder there is a further complication in that they have to regain existing rail height prior to reaching Rotherhithe New Road. In fact it is worse than that because you really want to regain height a few arches prior to the road to ensure equal forces on the bridge abutments on either side. In this particular case there is an added complexity in that the final three arches are the location of the historical former Southwark Park station which has been described in detail by IanVisits.

Work so far

The work at the site has been going on for a long time but was limited in scope until tracks started being taken out of use. Even then work that could be carried out was limited. On the SouthEastern side it was not the adjacent tracks (5 and 6) to the site that were initially taken out of use but lines 3 and 4. Lines 3 and 4 were then relaid and resignalled for future temporary use. Only then could lines 5 and 6 be removed and 3 and 4 brought back into service. This resignalling of lines 3 and 4 was made more complicated because trains are now running along them in the opposite direction to the way they were used to be before. As a further twist the signalling system was fooled into thinking that the the current (temporary) lines were actually 5 and 6 which meant that they were correctly recognised as the Charing Cross Up and Down lines.


Site of diveunder looking east towards New Cross

Whilst for a long time not much going on was visible and demolition only starting in earnest at the start of June, work was being carried out including constructing the concrete side walls adjacent to the diveunder box. This may not be a major construction task but it is an essential prerequisite to avoid a sheer drop from the existing trackbed once the site for the diveunder is cleared.

Wall of Diveunder box

Wall of diveunder box. Some of it is below tracked level

Since June demolition has commenced and proceeded at a rapid pace. The start of piling and construction of the diveunder box itself is imminent and will continue for the next year. By then the first trackbed using the diveunder to be brought into use will be ready – that of the down slow on the Brighton side from the London Bridge terminating platforms which uses the diveunder to emerge on the correct side of the future Thameslink Line. There are six months in the programme for tracklaying, signalling and testing. That line should open at the start of January 2017 and be the first significant bit of infrastructure to come into use since the current timetable restrictions were introduced. As Thameslink will continue to be diverted at that time its full benefit will not be realised, but it will mean a minimum of three tracks available all the way from London Bridge terminating platforms to New Cross Gate.

Later in 2017 the Charing Cross tracks will be diverted to their new permanent route using the diveunder. This then effectively frees the rest of the Southeastern part of the site for work to install the final layout on the Southeastern side during 2018. At the same time work will be continuing to get ready for use the new Thameslink route via London Bridge which will have its own dedicated tracks.


The start of 2018 will be notable as Thameslink services will be reinstated via London Bridge courtesy of the Bermondsey Diveunder. What is more, there will no longer be peak hour restrictions due to the elimination of the need to share platforms and tracks with Charing Cross trains. This will not be the end of the project. During 2018 the service through the centre of Thameslink will be increased in stages to provide the 24tph through the core by the time of the annual timetable change in December.

Track Layout

Track layouts before and after

Will it be worth it?

In a sense the question “will it be worth it?” is irrelevant, because it is going to happen and we are well beyond the point of no return. In one sense the Bermondsey Diveunder won’t achieve much on completion because the number of trains going to London Bridge is scarcely altered. In another sense it is a lot of gain for a relatively little pain because the number of trains going from London Bridge to Blackfriars, Farringdon and St Pancras in peak hours increase from 1 to 16. It is true fewer will go via Elephant & Castle but via London Bridge is where the demand is anyway. In financial terms it is a bargain given how much would have had needed to be spent anyway on this heavily used and fairly worn out part of our national railway. In time the benefits, which will be around for many years to come, will eclipse the three years of reduced and less reliable services and people will wonder why this wasn’t done sooner – but that is another story.

Dog need a drink

Scene outside Bermondsey Diveunder worksite entrance. Nothing to do with the Dog End at the nearby Den.

Our thanks for our hosts at Network Rail for making a site visit possible. Also ngh for the annotated photo and other helpful support.

Written by Pedantic of Purley