The Anonymous Widower

Will The Northern Line Extension Go All The Way?

There has been masses of reports about the Northern Line Extension in the special rail media and on the BBC and in other publications, like Time Out and the Standard.

Despite the long term aim of taking the extension to Clapham Junction, mentioned in the Wikipedia entry for the project.

Provision will be made for a possible future extension to Clapham Junction railway station by notifying the London Borough of Wandsworth of a reserved course underBattersea Park and subsequent streets

There has been no discussion about the extra station.

There has also been no statements from the consortium building the line about how they will actually construct the line.

As it is not the longest of tunnels, I suspect to save money, at the expense of possibly some extra time, they will use just one tunnel boring machine (TBM). It won’t be one of the ten-million-pounds-a-time  beasts used for Crossrail, as Underground tunnels are generally much smaller in diameter. Although, I wouldn’t be surprised to see the tunnels made slightly larger than normal for reasons of safety and perhaps to re-use a refurbished TBM from another project.

One thing that London Underground doesn’t like is lines that end deep in the ends of tunnels. For reasons of cost in the 1960s, the Victoria line is like this at both ends and according to Wikipedia, there have been proposals for creating a large reversing loop at Brixton to both reach Herne Hill and increase capacity.

Reversing loops also eliminate any possibility of a Moorgate disaster, which has still not been satisfacorily explained. But operationally they remove the need for trains to crawl into the end station for safety reasons, and reduce the time it takes to turn trains, thus increasing the frequency on the line.

Reversing loops with stations are not unknown in the UK. Terminal 4 at Heathrow is served by the Piccadilly Line in this way and the Merseyrail Loop Line, is a larger example, that reverses and provides several stations for the Wirral Line. It could also be argued that Bank station on the Docklands Light Railway is two platforms on a reversing loop.

So could a similar solution be used at Battersea to turn the trains? Look at this map of the area.

Around Battersea

Battersea Power Station and Battersea Park are obvious, but notice the Underground roundel marking Kennington station in the top-right corner and the British Rail symbol marking Clapham Junction station in the bottom-left.

The extension joins the current Northern line at Kennington and the Battersea station is proposed to be somewhere near the power station.

At present, Charing Cross branch trains reversing at Kennington, can go round the Kennington Loop. Wikipedia says this about the loop.

A loop tunnel south of the station enables southbound Charing Cross branch trains to be terminated at Kennington, leave the station in a southward direction and, traversing the loop, enter the northbound Charing Cross branch platform.

So instead of building two tunnels from Kennington to Battersea, with all the cost of two TBMs or the hassle of turning a single one round, I do wonder, if a cheaper and easier way of building the tunnels, would be to start in the Kennington Loop with a single TBM, tunnel via Nine Elms to Battersea and then create a wide reversing loop before returning to Battersea to dig the second tunnel back to the other side of the Kennington Loop.

One of the consequences of good project management is often that what the engineers build in the end is quite different, but better, more affordable and earlier to what the politicians said they wanted. For this reason alone, I wouldn’t be surprised if the Northern Line Extension is just a single tunnel, dug by a single refurbished TBM.

The cost savings are probably small change in a one-billion pound project, but the time spent inserting and removing a second machine, probably comes off the overall project time.

It then doesn’t take a great leap of the imagination to see that this construction/reversing loop could be extended so that it passes under Clapham Junction station.

There are other advantages too.

Operations, Safety and Reliability

I’ve mentioned the operational advantages of the reversing loop in quicker turn back of the trains and possible safety advantages, but as the extension is going to be a continuation of the Charing Cross branch of the Northern line and an extension of the Kennington Loop, there could be no complicated track work at Kennington. This will mean that the branch used through Central London, will determine the ultimate destination of the train. But this would allow London Underground to split the two lines and probably arrange dedicated platforms for the two northern branches at the operational nightmare that is Camden Town.

The reliability of two continuous lines would probably be a lot higher, than one that was constantly splitting and joining back again.

But whatever happens to the rest of the line, if the extension was a continuous reversing loop with no points or sidings, it could be built faster and would probably cost less, have a higher capacity and probably be more reliable.

The only problem would be if a train were to break down in the loop. But what happens on the Heathrow loop on the Piccadilly line?

Battersea and Nine Elms Stations

Digging the extension as a loop, also means that the two stations at Battersea and Nine Elms, become classic below ground stations of the Underground, like say Southgate and Manor House of the 1930s, and the modern Canary Wharf, where escalators and lifts descend to a wide lobby between the two lines.

Costs could even mean that they were identical below the surface, although architects would probably exercise some flare on the surface.

I also wonder if stations could be built with no escalators, but large efficient lifts, that were scheduled, so they went up and down in time with the trains.

Lift-only stations would be best as double-ended, with the lifts even coming up into car parks of the over-site development.

There is tremendous scope here for a good architect to build passenger-friendly and lower cost stations.

I’ve always believed that urban stations should have development on top, just like my local one at Dalston Junction. But how many stations anywhere make efficient use of expensive land?

Ventilation

A big problem with London Underground’s deep level lines, like the Northern, is keeping everything cool. All over London, you see structures like I photographed here. A continuous reversing loop must have advantages as all three stations would double as ventilation shafts, so there would be no need for any extra holes in the ground. If modern regulations mean that the tunnels have to be built with a walkway for evacuation, like those on the Docklands Light Railway, these larger diameter tunnels would probably help ventilation.

A secondary advantage of a well-ventilated reversing loop, is that it would be cool, so any trains on the loop could cool themselves down, just like many deep-level trains, do by basking on the surface in the suburbs.

Clapham Junction Station

In the future when the station at Clapham Junction is added, little or no tunnelling will be needed, as construction will probably involve sinking a shaft to link it to the current Clapham Junction rail station and creating a new platform or platforms alongside the reversing loop.

I would go for the single platform. In some ways then, this station would be like the Piccadilly Line station at Heathrow Terminal 4, which has a single platform on a one-way loop from Hatton Cross to Heathrow Central.

No surface buildings would be required and space would only be needed to sink the shaft during the construction phase.

Opening Clapham Junction station could give a problem in that some passengers will transfer off the trains from Basingstoke, Portsmouth, Southampton and other places to complete their journeys. Will the Northern Line cope?

On the other hand the new Clapham Junction station will link to Crossrail at Tottenham Court Road, so Clapham Junction would get a hopefully step-free link to Crossrail and on to Heathrow.

Platform Edge Doors

Regulations will probably mean that all the stations would have to be constructed with platform edge doors. A loop extension would probably have the minimum number of platform faces, thus subtracting another saving from the cost.

If I was designing the stations, all platform faces would be identical to save costs.

But if you went for lift-only stations, this could mean that a better design evolved, where the safety functions of the platform edge doors were achieved in better ways.

Train Stabling

A loop extension, if built in its simplest form, would not have any sidings for storing trains overnight or perhaps holding them when there was a problem on the Northern line.

But as the loop would of necessity be rather long between Battersea and Clapham Junction, the trains could be stabled or held in the loop. If staff needed to leave or join the trains and if the tunnel had a walkway, they’d just walk along to the nearest station.

Ease Of Construction

There has been a report in Global Rail News that the Northern Line Extension might be completed before Crossrail.

A loop extension leading off the Kennington Loop has implications for building the extension in a quick and affordable manner, so this might explain the optimism.

Consider the following.

1. No terminal station, platforms, junctions or sidings would need to be built.

2. As the main construction at Kennington only takes place on the Kennington Loop, trains on the Bank branch running to Morden will be unaffected. Trains on the Charing Cross branch would probably be suspended, unless some other way of turning them back could be found.

3. One TBM digs the whole tunnel in a continuous operation.

4. The project length is determined by the time to dig the tunnel, fit it out with track and signalling, connect it to the existing network and then give it an extensive testing, as once the TBM has passed, the construction of the platforms can be started in parallel with the rest of the project.

5. It would probably be easier to adjust the route of a loop tunnel to avoid other infrastructure and the proposed route of Crossrail 2. Victorian engineers didn’t have machines of the accuracy of today’s modern TBMs.

6. Crossrail needed a lot of shafts to insert and extract the TBMs and other equipment and materials. I’m no expert, but surely material could be brought in by service trains on the Morden branch of the Northern line, to avoid digging too many shafts.

Conclusion

I strongly believe the first phase of the Northern Line Extension will be built as a loop off the Kennington Loop and include all the tunnelling for the extension to Clapham Junction.

Clapham Junction station would not look like a terminus, as it would only be a single platform on a reversing loop.

So the second phase would solely be the fitting out of the station and connecting it to the rail station.

 

November 15, 2014 - Posted by | Transport | , , , , , ,

2 Comments »

  1. […] writing my piece about the Northern Line Extension, I went to have a look at Kennington […]

    Pingback by A Heritage Station With Four Clocks « The Anonymous Widower | November 16, 2014 | Reply

  2. […] I said this in my piece on the Northern Line Extension. […]

    Pingback by Are There Any Other Places Where A Loop Extension With Stations Can Be Built? « The Anonymous Widower | November 16, 2014 | Reply


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