Should a Severn Rail Bridge Be Built?

[Envoy]

Split from

Severn Tunnel Closure Summer 2026

I noticed on GWR website that there'll be rail improvement work between Bristol to South Wales between 18-May and 15-June?! That is much longer than expected as I remembered only 2 weeks (or shorter) in recent years. I can't imagine they'll close it for 4 weeks! Maybe I am overreacting as it...

www.railforums.co.uk

This is getting beyond a joke. Time to consider a new crossing but surely a bridge would be liable to salt spray?

 

[Harpo]

This is getting beyond a joke. Time to consider a new crossing but surely a bridge would be liable to salt spray?

The greatest opportunity was missed with the second Severn bridge which could have had a rail deck. As for salt spray, the Forth bridge seems to have done alright? Plus paint of course.

 

[Torfaen]

The greatest opportunity was missed with the second Severn bridge which could have had a rail deck. As for salt spray, the Forth bridge seems to have done alright? Plus paint of course.

Agreed. The Prince of Wales Bridge is on a very similar alignment to the tunnel (it actually crosses it near the English side).
I would have thought the effect of salt spray on a bridge (particularly one high enough for ships to pass underneath) would be less than for a railway that closely follows the coast.

 

[The exile]

The greatest opportunity was missed with the second Severn bridge which could have had a rail deck. As for salt spray, the Forth bridge seems to have done alright? Plus paint of course.

It is stuff of legend that Forth Bridge was under continuous painting (let alone other maintenance) at least until BR / Railtrack decided it was unnecessary - with quite disastrous consequences. The difference being that the bridge didn’t need to be closed for that work to proceed.

 

[The Planner]

You have to remember that the second Severn crossing would have started design etc in the late 80s, so 40 years ago. The railway wasn't likely to get the funding as it was still in decline.

 

[brad465]

Split from

Severn Tunnel Closure Summer 2026

I noticed on GWR website that there'll be rail improvement work between Bristol to South Wales between 18-May and 15-June?! That is much longer than expected as I remembered only 2 weeks (or shorter) in recent years. I can't imagine they'll close it for 4 weeks! Maybe I am overreacting as it...

www.railforums.co.uk

This is getting beyond a joke. Time to consider a new crossing but surely a bridge would be liable to salt spray?

A bridge will of course have its own maintenance costs, but you also wouldn't have the cost of running pumps continuously on top of this repeated extra maintenance that electrification and age is causing. While there will be a long return period on investment, I feel a bridge would be a return on investment if the maintenance costs are lower and at the same time more revenue-earning traffic is able to use it compared to the tunnel.

 

[Class 317]

Any new Severn crossing should be designed to standards for and to line up with any future HSL from London. I know this seems unlikely now.

For the small benefits through, the cost would be huge so it's probably not worth it. An alternative which is likely to be more affordable would be to upgrade the diversion via Gloucester to allow better services during closures particularly if these are becoming more frequent.

A more pressing concern in the area is likely to be the flooding between Swindon and Bath / Swindon and Bristol Parkway.

 

[Tetchytyke]

A bridge will of course have its own maintenance costs, but you also wouldn't have the cost of running pumps continuously on top of this repeated extra maintenance that electrification and age is causing.

Swings and roundabouts, I'd say. A Severn bridge wouldn't have the same maintenance costs as the tunnel does, but it would have separate challenges of its own. Look how often the Severn road bridges are closed due to high winds.

 

[brad465]

Swings and roundabouts, I'd say. A Severn bridge wouldn't have the same maintenance costs as the tunnel does, but it would have separate challenges of its own. Look how often the Severn road bridges are closed due to high winds.

Doesn't the Second Severn Crossing have wind shields on it that reduce closures? If a rail-equivalent can be developed this won't be as much of an issue.

 

[Sun Chariot]

Doesn't the Second Severn Crossing have wind shields on it that reduce closures? If a rail-equivalent can be developed this won't be as much of an issue.

The Prince of Wales (M4) Severn Crossing indeed has wind-deflectors to shield cars and low sided vehicles. High-sided vehicles + those with large cross-section (think "sail effect") are the types typically prevented in times of risky crosswinds.

A fully-enclosed railway line obviates that problem. Wait - we have one of those already.

 

[brad465]

The Prince of Wales (M4) Severn Crossing indeed has wind-deflectors to shield cars and low sided vehicles. High-sided vehicles + those with large cross-section (think "sail effect") are the types typically prevented in times of risky crosswinds.

A fully-enclosed railway line obviates that problem. Wait - we have one of those already.

But that fully-enclosed railway is now running up some extensive maintenance costs, both raw maintenance and economic costs from closing it for long periods. Of course if a brand new tunnel could be bored under the estuary that is watertight and is long-term more economical than a bridge, go for it. But until then the current setup does not look very sustainable.

 

[SynthD]

A tunnel lasts longer than a bridge, which may kill off the idea that there is a return of investment in moving to a bridge. I doubt there are two similar enough structures to compare, but what’s the downtime the tunnel has had in its life compared to similar aged bridges like the Monkwearmouth or Kew railway bridges?

 

[The exile]

A tunnel lasts longer than a bridge, which may kill off the idea that there is a return of investment in moving to a bridge. I doubt there are two similar enough structures to compare, but what’s the downtime the tunnel has had in its life compared to similar aged bridges like the Monkwearmouth or Kew railway bridges?

Tay or Forth Rail Bridges would be a better comparison, surely?

 

[Sun Chariot]

Of course if a brand new tunnel could be bored under the estuary that is watertight and is long-term more economical than a bridge, go for it. But until then the current setup does not look very sustainable.

The investment required for a new tunnel, will need a political appetite and influence; I suspect is not a "vote-winner" sadly.

Crossrail's tunnelling contracts (awarded December 2010 + April 2011) were just shy of £1.5 billion. Factor in 16-years of cost-increases in labour, energy and materials - and the new Severn Tunnel, fabulous as it would be, is a pricey beast.

 

[zwk500]

Tay or Forth Rail Bridges would be a better comparison, surely?

Or the original Severn Rail Bridge, which has had 65 years of very literal downtime.

Severn Railway Bridge - Wikipedia

en.wikipedia.org

But that fully-enclosed railway is now running up some extensive maintenance costs, both raw maintenance and economic costs from closing it for long periods. Of course if a brand new tunnel could be bored under the estuary that is watertight and is long-term more economical than a bridge, go for it. But until then the current setup does not look very sustainable.

A bridge will need to be high enough to meet clearance requirements for Sharpness and Gloucester, as well as having a main span long enough for the biggest width of boat. I don't know whether the changes in marine traffic mean that could be kept lower than either Motorway bridge if sufficiently far upstream, but if it needs to be lifted clear then having an enclosed tube is going to give the wind a big flat surface to hit. Even wind baffles aren't 100% effective and would need maintenance.
On the flip side, the gradient in the tunnel to clear the bed of the estuary is essentially 1:100 downhill from Patchway down to the midpoint (although the normal downhill track has steeper points, due to being an older design), then 1:100 up from the Midpoint to Severn Tunnel Junction. You can certainly hear diesel trains working hard on the climb up from the tunnel to Pilning. A modern tunnel may be able to lessen the clearances required to the estuary floor and start from a different point to minimise gradients but there's a limit of what can practicably be done as the main navigation channel will need to be cleared.
There's no easy answer, unfortunately. A bridge upstream of the M48 somewhere would likely make the best economic case as it's more sheltered than out in the open estuary, is spanning a narrower part of the river, would have less intense gradients, would not need the same restrictions on 2 trains being in the tunnel at the same time for evacuation/train rescue purposes and is upstream of the main docks for the really big ships.

 

[The Planner]

There is of course this old thread

Hypothetical Severn Tunnel Replacement

Assuming there was a major structural disaster with the Severn Tunnel that was beyond economical repair, which replacement option would be the most cost effective? 1. Build an entirely new tunnel to modern standards. 2. Build a rail bridge over the Severn. 3. Build a smaller crossing further...

www.railforums.co.uk

If you believe Google AI, then it reckons its £1 million a year in normal maintenance. Would a bridge pay this back including inflation and increased maintenance of the tunnel over 100 years?

The Prince of Wales bridge came in at £330 million in 1996, add normal inflation (not construction) and that is £671 million now. I bet you are looking at a billion plus for a new bridge.

 

[Sir Felix Pole]

Or the original Severn Rail Bridge, which has had 65 years of very literal downtime.

Severn Railway Bridge - Wikipedia

en.wikipedia.org

A bridge will need to be high enough to meet clearance requirements for Sharpness and Gloucester, as well as having a main span long enough for the biggest width of boat. I don't know whether the changes in marine traffic mean that could be kept lower than either Motorway bridge if sufficiently far upstream, but if it needs to be lifted clear then having an enclosed tube is going to give the wind a big flat surface to hit. Even wind baffles aren't 100% effective and would need maintenance.
On the flip side, the gradient in the tunnel to clear the bed of the estuary is essentially 1:100 downhill from Patchway down to the midpoint (although the normal downhill track has steeper points, due to being an older design), then 1:100 up from the Midpoint to Severn Tunnel Junction. You can certainly hear diesel trains working hard on the climb up from the tunnel to Pilning. A modern tunnel may be able to lessen the clearances required to the estuary floor and start from a different point to minimise gradients but there's a limit of what can practicably be done as the main navigation channel will need to be cleared.
There's no easy answer, unfortunately. A bridge upstream of the M48 somewhere would likely make the best economic case as it's more sheltered than out in the open estuary, is spanning a narrower part of the river, would have less intense gradients, would not need the same restrictions on 2 trains being in the tunnel at the same time for evacuation/train rescue purposes and is upstream of the main docks for the really big ships.

The M48 Severn Road Bridge is going to need replacing at some point - it is not available to HGVs and has the ongoing corrosion problem of the main suspension cables. A combined road / rail bridge here is one solution, but it would require significant new rail construction on the Gloucestershire side to link up with the SWML.

Has the source of the 'Great Spring' in the tunnel ever been identified -and could it be diverted away?

 

[lachlan]

The M48 Severn Road Bridge is going to need replacing at some point - it is not available to HGVs and has the ongoing corrosion problem of the main suspension cables. A combined road / rail bridge here is one solution, but it would require significant new rail construction on the Gloucestershire side to link up with the SWML.

Has the source of the 'Great Spring' in the tunnel ever been identified -and could it be diverted away?

A set of sliproads connecting the westbound M4 with eastbound M48 and vice-versa would be easier than building a new bridge.

 

[HSTEd]

I would think a cable stay or extradosed rail bridge would be a major improvement over the existing tunnel structure.

Higher speed, higher capacity (due to not requiring any signalling limitations) and avoiding the constant annoyances of the tunnels peculiar maintenance needs.

I expect the two options that would be most suitable for consideration would be a bridge and an immersed tube tunnel.

EDIT:

250kph capable multi track railways have been built over cable stayed bridges with 1100m class spans in China. Indeed its been done so many times its almost routine there now!

 

[Snow1964]

Could a new single track tunnel be bored using a modern TBM, then the existing tunnel single tracked to allow additional lining.

Although might be easier to use a bore a larger double track tunnel, so can temporarily close old one to add inner lining, before single tracking it. Having a third tidal flow track in long term will probably help with track capacity.

 

[The Planner]

The M48 Severn Road Bridge is going to need replacing at some point - it is not available to HGVs and has the ongoing corrosion problem of the main suspension cables. A combined road / rail bridge here is one solution, but it would require significant new rail construction on the Gloucestershire side to link up with the SWML.

Has the source of the 'Great Spring' in the tunnel ever been identified -and could it be diverted away?

It won't get replaced, it will end up like the original Forth Road bridge. Buses, taxis and cycles.

 

[HSTEd]

It won't get replaced, it will end up like the original Forth Road bridge. Buses, taxis and cycles.

The situation is rather different, in that the Second Severn Crossing is already approaching its design capacity and I believe traffic is still growing.

Significant numbers of vehicles (15000+/day) still pass over the old bridge, if it was to (functionally) close like the Forth Bridge they'd be creating a substantial overload on the new crossing.

I think its more likely that a new crossing will be required.

 

[Pigeon]

Has the source of the 'Great Spring' in the tunnel ever been identified -and could it be diverted away?

It's the combined drainage from several square miles on the Welsh shore percolating down through porosities and fissures in the rock. It can't be diverted away because there is nowhere to divert it to. Before the tunnel came along and intersected the fissure system it stood full of water up to the surface, and various water features on the surface no longer exist since the tunnel drained it all out from below.

They tried to block it off when they were building the tunnel but it was no go. Once the tunnel had tapped the system whatever hydrostatic head existed in the system was applied to the whole outside of the tunnel structure over a considerable distance, and as the water level of the blocked-off spring rose again the brickwork began to fail under the pressure, with jets of water coming in and bricks being pushed out or simply failing in compression and spalling bits off. So they binned that idea and turned the pumps back on again.

IIRC the clearance inside the tunnel is somewhat larger than normal, and it may be possible that these days some combination of internal concrete lining and external grouting could provide enough reinforcement and sealing to do what was not possible then. But it wouldn't be a weekend project.

I reckon they should have binned the whole project at that point, or earlier, ie. instead of reviving the derelict works. Nearly all the previous schemes for crossing the Severn estuary had been bridges, and the first scheme to succeed was a bridge, authorised in the same session as the tunnel but opening something like 9 years earlier at about the time the spring broke in. Then only a couple of years after the tunnel, the Forth bridge was opened. To cross the Severn from rock-shelf to rock-shelf doesn't require that much more of a maximum span than the Forth bridge, so I don't think doing something similar across the Severn would have been too much of a stretch.

 

[JKF]

Could they add a rail deck underneath the POW bridge? There’s a precedent for this in Lisbon, where rail was added to the 25th April bridge over the Tagus in 1999, 33 years after it was built as a road bridge.

 

[PTR 444]

If such a crossing were to be a bridge, it should ideally be located north of the M48 road bridge to reduce the cost and weather impact on it. In terms of the route taken, I’d look at reopening the Thornbury Branch Line and extending it past Oldbury to cross the Severn, before joining up with the Gloucester - Newport line at a 3 way junction near the Severnside Equestrian Centre.

While this wouldn’t become the main route for fast London - Cardiff and Bristol - Cardiff trains, it would act as an important local route linking Bristol with Thornbury, Chepstow and Lydney, as well as a diversionary route for whenever the Severn Tunnel or Cross Country route is blocked.

 

[Sir Felix Pole]

It's the combined drainage from several square miles on the Welsh shore percolating down through porosities and fissures in the rock. It can't be diverted away because there is nowhere to divert it to. Before the tunnel came along and intersected the fissure system it stood full of water up to the surface, and various water features on the surface no longer exist since the tunnel drained it all out from below.

They tried to block it off when they were building the tunnel but it was no go. Once the tunnel had tapped the system whatever hydrostatic head existed in the system was applied to the whole outside of the tunnel structure over a considerable distance, and as the water level of the blocked-off spring rose again the brickwork began to fail under the pressure, with jets of water coming in and bricks being pushed out or simply failing in compression and spalling bits off. So they binned that idea and turned the pumps back on again.

IIRC the clearance inside the tunnel is somewhat larger than normal, and it may be possible that these days some combination of internal concrete lining and external grouting could provide enough reinforcement and sealing to do what was not possible then. But it wouldn't be a weekend project.

I reckon they should have binned the whole project at that point, or earlier, ie. instead of reviving the derelict works. Nearly all the previous schemes for crossing the Severn estuary had been bridges, and the first scheme to succeed was a bridge, authorised in the same session as the tunnel but opening something like 9 years earlier at about the time the spring broke in. Then only a couple of years after the tunnel, the Forth bridge was opened. To cross the Severn from rock-shelf to rock-shelf doesn't require that much more of a maximum span than the Forth bridge, so I don't think doing something similar across the Severn would have been too much of a stretch.

Interesting, thanks. I guess a 'Forth Bridge' arrangement would have required lengthy approach viaducts on the shores to get the necessary height for navigation clearance - something like the Rendsburg bridge over the Kiel Canal in Germany maybe.

 

[MarkyT]

IIRC the clearance inside the tunnel is somewhat larger than normal, and it may be possible that these days some combination of internal concrete lining and external grouting could provide enough reinforcement and sealing to do what was not possible then. But it wouldn't be a weekend project.

Relining might be useful to secure a long term future for the structure after a new crossing took the traffic away. The old tunnel might then be reintroduced following full modernisation with a new lining, a single track, and evacuation platform alongside. The old tunnel could supplement a new double track bore or pair of single bores to produce a flexible 3-track railway.

Then only a couple of years after the tunnel, the Forth bridge was opened. To cross the Severn from rock-shelf to rock-shelf doesn't require that much more of a maximum span than the Forth bridge, so I don't think doing something similar across the Severn would have been too much of a stretch.

The Forth Bridge has the advantage of high ground to launch from on both sides, avoiding long ramps on approach to clear shipping.

Could they add a rail deck underneath the POW bridge? There’s a precedent for this in Lisbon, where rail was added to the 25th April bridge over the Tagus in 1999, 33 years after it was built as a road bridge.

The Tagus bridge was designed with that in mind from the start though. Connecting railways were not constructed at the same time as the bridge, though they were fully intended as a future upgrade. Economies were forced on the original project however, so some support cables were omitted to suit the revised road-only loads.

A full duplicate set of cables, hanging from raised towers had to be retrofitted for the eventual rail project along with other measures. I recall that, even in the late 1990s, using a suspension bridge for a heavy rail link was considered groundbreaking.

 

[The Planner]

If such a crossing were to be a bridge, it should ideally be located north of the M48 road bridge to reduce the cost and weather impact on it. In terms of the route taken, I’d look at reopening the Thornbury Branch Line and extending it past Oldbury to cross the Severn, before joining up with the Gloucester - Newport line at a 3 way junction near the Severnside Equestrian Centre.

While this wouldn’t become the main route for fast London - Cardiff and Bristol - Cardiff trains, it would act as an important local route linking Bristol with Thornbury, Chepstow and Lydney, as well as a diversionary route for whenever the Severn Tunnel or Cross Country route is blocked.

That makes no sense, if you build a new crossing you make it as fast and short as possible. You aren't going to build a new crossing as a secondary route.

 

[Pigeon]

Interesting, thanks. I guess a 'Forth Bridge' arrangement would have required lengthy approach viaducts on the shores to get the necessary height for navigation clearance - something like the Rendsburg bridge over the Kiel Canal in Germany maybe.

Yes, but on the other hand most of the length of the tunnel is "approach tunnel" to get the depth required to pass a safe distance below the deepest point of the river channel. At low tide this short bit is the only part which is actually under water, so only the corresponding part of a bridge would need to provide the full clearance. I assume that the required clearance would be somewhere between 100ft (Saltash) and 150ft (Forth), so it would have to go up about the same distance as the tunnel goes down, so with the same gradient on the inclines you end up with the same overall length. A good deal of that you'd want to be viaduct in any case, to cross the intertidal areas.

 

[Torfaen]

Indeed - just like the PoW bridge itself. I don't imagine you'd need anything steeper than the existing 1 in 90 and 1 in 100 gradients approaching the tunnel.