The Anonymous Widower

Could We Just Double The Width Of The Digswell Viaduct?

The Digswell Viaduct and the associated double-track railway through Welwyn North station at its northern end, on the East Coast Main Line is probably one of the biggest bottlenecks on railways in the UK. Wikipedia says this about the Grade II* Listed viaduct.

The viaduct carries the East Coast Main Line, which has to narrow from four tracks to two to cross the viaduct, making it a bottleneck restraining capacity over this strategic transport route. This problem is exacerbated by Welwyn North railway station situated at the northern end of the viaduct, which blocks the line while trains are stationary and two tunnels to the north. Several ideas to overcome the limitations of the viaduct and station without damaging the viaduct’s essential historic character and rhythmic design are periodically discussed. The line was electrified in the 1970s.

Various plans have been put forward to remove the bottleneck cause by this masterpiece of Victorian engineering.

The Current Capacity

Network Rail have published this report, which is entitled The Capacity Of The Welwyn Viaduct. This is said about the capacity of the twin-track section.

The two track section between Woolmer Green Junction and Digswell contains both Welwyn Viaduct and Welwyn Tunnel and is approximately 2.5 miles in length.

In pure theoretical terms the capacity of the viaduct is dictated by the headway over the section.

There is a planning headway of 3 minutes over the two track section which therefore results in a theoretical maximum capacity of 20 trains per hour. To achieve this capacity would require a fully homogeneous service (for example same rolling stock and calling patterns) and 100% use of planning capacity.

The usable capacity is below the theoretical and is determined by the service specification which needs to use the capacity. The current and future specifications for the section require calls at Welwyn North Station which is on the two track section. This reduces the number of paths that can be achieved in a single hour over the viaduct. The usable capacity is also determined by the fast line capacity between Finsbury Park and Digswell and the difference in speed of rolling stock approaching the two track section which will determine whether trains can be flighted over the viaduct at 3 minute slots to achieve the theoretical capacity.

There is no defined permitted number of paths on the viaduct as the capacity available is a function of demand and therefore the type and number of services which need to use it.

Network Rail concludes that eighteen trains per hour is a theoretical maximum on the current track layout.

Reason For Removal Of The Bottleneck

Whether or not HS2 is built, the East Coast Main Line must be improved to handle the large and ever growing traffic between London and Leeds, Newcastle and Edinburgh.

If more of the line was four-track, it would make the train companies aim of a frequent four-hour service to Edinburgh achievable. It could be even faster, if a lot of the line could handle trains at one hundred and forty mph, rather than the current one hundred and twenty-five.

Proposed developments are detailed in Wikpedia.

One of the most important is removing the bottleneck at Welwyn.

One Problem Or Two?

I think that when outsiders look at this bottleneck, they see one problem, but I think it is fair to describe it as two.

  • Welwyn North station and the tunnels to the North.
  • The Digswell Viaduct itself

In my view both problems need their own solutions.

We shouldn’t also forget other smaller changes, that can take the pressure of the area.

  • More and better use of an upgraded Hertford Loop Line.
  • More precise and better timetabling of trains.
  • As Thameslink beds down, we’ll see other improvements.

I also wonder, if a fully-electrified freight route could be created between Peterborough and London, through March, Ely and Cambridge, that used the extra capacity of a four-tracked West Anglia Main Line.

Welwyn North Station

In some ways the station is as big a bottleneck as the viaduct, as the two trains an hour that stop in the station, effectively block the line for a few minutes.

It is also one of those heritage problems, that Network Rail love so much. This is said in the Wikipedia entry for the station.

The station is a rare survival of architecture from the early days of the GNR and this is now recognised with listed building status. The main station building, the footbridge, the tunnel portal to the north and Welwyn Viaduct to the south are all Grade 2 listed.

So I doubt that modifying the station will be easy.

This Google Map shows the lines through the station.

Welwyn North Station

Welwyn North Station

These are some pictures of Welwyn North station taken on another day.

The images,  probably shows another problem in that four-tracking the line through Welwyn North station would probably close the car parks.

The Digswell Viaduct

The Digswell Viaduct is an iconic structure and if the views of the viaduct and the valley it crosses were to be altered in any negative way, there would be a battle that would make the protests over HS2 look like a child’s tea party.

Around 1890, they had a similar capacity problem at the Stockport Viaduct, which was successfully widened from two tracks to four.

It is my view, that with major advances in structural engineering and construction methods, that widening the viaduct would be one of the better methods to improving the capacity through the area, without changing the look of the viaduct.

Intriguingly, if the East Coast Main Line was not already electrified, with the recent development of IPEMU-technology, I suspect now that Network Rail would think seriously about not electrifying the viaduct.

Trains would cross using their on-board energy storage, raising and lowering their pantographs appropriately.

Knebworth Station

This Google Map shows Knebworth station, a few miles to the North of Welwyn North.

KnebworthStation

This station has four platforms arranged on two islands.

For comparison, this is an image of Welwyn North station to the same scale.

A1

I think that four-tracking Welwyn North station will be a tight fit.

Comclusion

At some point, I feel that Network Rail will bite the bullet on four-tracking this section of line and the fight will be a big one.

 

March 14, 2016 - Posted by | Travel | , ,

1 Comment »

  1. Ah, “just”. The wallpaper word.
    Apparently to cure damp at Lincoln cathederal, someone suggested “just put a waterproof membrane under it”.

    Comment by Conrad McMahon | July 23, 2018 | Reply


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