If you want a good example of why the Lower Thames Crossing should be multimodal and include rail you only have to look at the sad case of the Second Severn Crossing, Built as car only it simply shifts the constraint of congestion up a couple of motorway junctions to Newport, where road options have been rejected. The plan now is to invest in improved rail in South Wales, following the report of the South East Wales Transport Commission, great, but now the constraint is rail capacity.
You might not have heard of the Western Powerhouse, that is because it so far has no firm plans, not even a chief executive, it has recently advertised for one. The broad vision is to better connect South Wales and the West of England/Gloucestershire/Wiltshire. The broad ambition is to link the metro systems of Cardiff and Bristol (that is if Bristol had one, one of the reasons the West of England Mayoralty was lost because of lack of progress here).
There is a problem, cross Severn rail is at capacity. There simply is no spare capacity to run both intercity and short hop services through the Severn Tunnel. From Swansea to Caldicote via Cardiff there is a fantastic four track line making it ideal as the backbone of a fast metro service. However then the line splits to either go through two tracks under the Severn Tunnel and two track to Chepstow and then on to Gloucester, it having lost all of its intermediate stations many years ago.
Even if the Victorian Severn Tunnel, which is expensive to maintain and prevent flooding, was for tracks the route into Bristol is only two track and narrowing to one.
There is solution, a new rail tunnel from Avonmouth to Magor, a distance of about three miles, the reason being there is a two track width (sadly only one track remains, but two track bed width remains) from Avonmouth to Bristol Temple Meads, and tram trains could switch to street running at Bristol Docks and loop back to Temple Meads.
This would logically fit within the likely recommendations of the Hendry Commission to improve links between England and Wales.
Gloucestershire CC is looking at a rail bridge between Sharpness (where a new settlement is planned by the Port) and Lydney, but this is a red herring as it is far too much of a dog leg for an intracity metro. I can see a case for a road bridge between Epley and Rodley however, linking the M5 and A48, to avoid all Forest of Dean traffic falling on the A48 /A40 Junction which has scuppered one and maybe 2 new settlement options in teh Forest of Dean (outside teh Forest).
The starting point has to be for the Western Gateway partnership, GCC, West of England CE and the Welsh Government to pool there efforts, form one ‘Transport for the West’ authority on the TfN model and undertake some serious studies. There is also considerable potential to open new rail stations on links from Gloucester to Bristol and Gloucester to Forest of Dean Lines, which could act as nodes for new /expanded settlements, some of which are planned (as in Stround district) solely to be accessible form the M5, hardly zero carbon.
As I have said before the Severn Vale/Western Gateway offers to showcase the potential for integrated land/use/transport strategic planning like no other areas. You can protect the most sesnitive areas like the Forest of Dean (new National Park) and the Cotswolds, whilst unlocking major areas for development, level up the valleys and make employment areas like the Llantwern steel works site accessible, whilst avoiding new roads across the Gwent Levels.