DIGSWELL VIADUCT (Welwyn) Double Decking for 4 tracks?

[Alpine Rider]

Welwyn station and Digswell viaduct are regularly cited as a bottleneck on services. Unfortunately the existing viaduct structure has been listed which pretty much eliminates suggestions of widening the current structure to accommodate 2 extra tracks which it seems is really needed to uncork that bottleneck.

I note that despite the "listing" at some stage overhead line equipment has been bolted to every sixth arch of the existing structure without the world coming to an end. So I wonder if Network rail could go a bit further and arrange to bolt more a lot more metal work onto the viaduct sides to effectively roof over the existing running tracks and support two additional tracks (with OHLE) on the upper deck.

The current brick build viaduct does seem to have all the hallmarks of many over engineered Cubbit structures - but doubtless someone can do the math to determine whether the foundations/structure could support a bit more static load. I can find no record of any of Cubbits structures failing - save due to WW2 bombs!

Stopping services could remain on the lower deck and call at Welwyn North as usual but without delaying through services running on the upper deck? Obviously a new two track tunnel would also be needed for the upper deck services along with some grade separation to create a 4 track route over the Mimram Valley.

Modern materials have allowed several double deck bridges to be constructed around the planet, so why not a double deck viaduct which hopefully could be craned into place with little or no impact on existing services?

 

[D365]

On an entirely hypothetical basis, and noting the subforum that this has been posted under.

Would a double deck viaduct + associated tunnels offer any advantage when compared to a Welwyn bypass tunnel (as discussed in the recent past).

 

[quantinghome]

On an entirely hypothetical basis, and noting the subforum that this has been posted under.

Would a double deck viaduct + associated tunnels offer any advantage when compared to a Welwyn bypass tunnel (as discussed in the recent past).

I guess it would limit land take, but the disadvantages would massively outweigh this.

ISTR that when this bottleneck came up in previous discussions, it wouldn't enable that many more service due to other bottlenecks on the ECML. I guess capacity at KX would be the killer.

 

[HSTEd]

I can't see this not producing howels of protest that would make the complaints over two tunnel entrances and a couple of intervention shafts pale in comparison. Plus I would worry you will struggle to get the approach ramps in at the north end, given Welwyn North is in the way and its not that far to the tunnels.

I think the lesson of the last couple of decades is that engineering problems are far easier to solve than political/sociopolitical ones.

 

[Magdalia]

Unfortunately the existing viaduct structure has been listed

It isn't unfortunate. Here is a picture.

cl 700 Welwyn viaduct 1242 BTN-CBG 6-8-22

On 6 August 2022, a 700 crosses Welwyn viaduct on the 1242 Brighton - Cambridge. The yellow grass is evidence of the lack of rain.

www.flickr.com

cl 700 Welwyn viaduct 1242 BTN-CBG 6-8-22​

On 6 August 2022, a 700 crosses Welwyn viaduct on the 1242 Brighton - Cambridge.​

The viaduct is not the only listed structure on the 2 track section. The Welwyn North station building, the Welwyn North station footbridge, and the south portal of Welwyn South tunnel (the portal nearest to Welwyn North station) are all listed too.

Obviously a new two track tunnel would also be needed for the upper deck services along with some grade separation to create a 4 track route over the Mimram Valley.

None of those are trivial. You have also given no explanation of what you are going to do with the extra tracks between the north end of the viaduct and the south tunnel, a section that includes Welwyn North station, with its listed station building and listed footbridge.

so why not a double deck viaduct which hopefully could be craned into place with little or no impact on existing services?

The viaduct is 30 metres/100 feet high. That's a very big crane. What are you going to stand it on?

ISTR that when this bottleneck came up in previous discussions, it wouldn't enable that many more service due to other bottlenecks on the ECML. I guess capacity at KX would be the killer.

This is right. 4 tracks at Digswell do not gain any additional capacity without additional capacity at Kings Cross too.

 

[edwin_m]

A masonry structure in good condition and with firm foundations can usually tolerate a load well in excess of what it actually carries, so there might be no engineering issue in building a structure on top of it to carry extra tracks. However, the approach structures to bring two tracks to a higher level and align them onto the viaduct itself without restricting their speed would be absolutely horrendous, due to the exceptionally shallow skew angles. Welwyn North station would disappear under one of these.

Putting a wider deck on the existing structure would be less difficult and less obtrusive than adding a deck above, and would allow all tracks to remain at the same level thus reducing the work needed each end. Not that that would make it easy or acceptable.

 

[Bald Rick]

Welwyn station and Digswell viaduct are regularly cited as a bottleneck on services.

and not entirely accurately.

I note that despite the "listing" at some stage overhead line equipment has been bolted to every sixth arch of the existing structure without the world coming to an end.

The listing occurred a couple of years after the OLE was erected. The OLE is technically part of the listing.

Would a double deck viaduct + associated tunnels offer any advantage when compared to a Welwyn bypass tunnel (as discussed in the recent past).

Well, if you count annoying most of the locals in the vicinity of the viaduct and Welwyn North as an advantage, then yes