So far 2026 from Euston to Birmingham Lichfield has turned into (at best) 2030 OOC to Birmingham and 2032 OOC to Lichfield. Its not looking like a success. If it looks successful upon completion it doesn't follow that a new line would be worth it from an economic or political perspective. There are likely to be better, lower risk, transportation and decarbonisation projects to fund. If you gave MPs the option to go back in time and go for campaign to protect rural England's 2009 proposal to have lines from London to Rugby and Leicester (hugging motorway) then most would take it. Id like HS2 to be a success but unless it is turned around very soon then future HS lines are a fantasy.
There's only so many upgrades you can make to the existing rail network, we've made a significant number of them (or will have by 2040). That limits what you can do there.
At some point that means new lines.
Given that a lot of the capacity constraints are within the urban areas, if we wish to increase capacity chances are that's likely to mean tunnels of some variety. Now that could tunnels for your metro services to allow your long distance services to use the existing stations (like would happen with Crossrail 2 allowing Southampton, Salisbury, and others to see more frequent services into Waterloo). Alternatively you look at copying HS2 and building new lines into the urban areas.
The issue with the former is that you potentially limit your train lengths OR you reduce capacity by needing longer sections (not necessarily a bad thing as it'll give you a bit more resilience as you'll not be quite so right) for the longer trains OR it costs to resignal the area (again not necessarily a bad thing, as it could be part of your digital signals which aren't fixed blocks which then gives you more capacity).
You also potentially have to build more tunnels, (now that could be justified because of the improvements to the metro services) as the metro lines would probably need to connect to several different areas rather than one or two (if its a through route). Whilst the tunnel size could be smaller for metro lines, there's a school of thought that you build one larger tunnel and double stack the lines (say eastbound above westbound) and then use the extra width for platforms and rolling stock storage (between stations) as that removes the (high) cost of the station boxes.
The issue with following motorways is that there's often a lot which you have to avoid around them (not least the junctions with the rest of the network, as well as development which has been attracted there by the motorways - not everywhere, but enough it's not as straightforward as just following a motorway).
The curve ball suggestion is you remove a lane of the motorway in each direction and build a railway within that space. However, that means either motorway junctions are very complex or moving the railway away (such as to build a station or to connect to another part of the network) gets complex (read costly). However you also need to have already reduced road traffic to allow this to happen.
Either route following a motorway assuming that you've got reasonable railway sizes bends built into the motorway (which aren't always a given) to allow reasonable speeds (we don't want to be building ANY railway which means reducing speeds to much less than 100mph. As by going slower it increases the number of trains needed.
This is demonstrated by the fact that HS2 to Manchester needs fewer coaches than running the existing services, even though the average number of coaches increases by 60% (9 or 11 coaches to 16 coaches).
The maths is 10 coaches (the average of 9 coaches or 11 coaches) taking 5 hours to do the round trip ready to run again, for 3tph that's 150 coaches. Conversely 16 coaches taking 3 hours for 3tph that's 144 coaches. That's a lot of extra capacity without any extra day to day costs (actually with the faster journey times you also reduce staff costs too).
Part of the issue with HS2 is that it's very easy to sell that all land with plants on is equally good and so anything which destroys our countryside is bad all the time it's within the countryside. For example if you had to route something through a wetland or ancient woodland which is likely to be more important ecologically and from a carbon capture perception?
Trees are good, ancient trees more so, therefore the ancient woodland, right?
It's not always that straightforward. For example wetlands (especially bogs) can absorb more carbon than almost any other habitat type. As if we lost all our wetlands in the UK, we couldn't plant enough trees to compensate for their loss - even if we had all the land in the UK at our disposal.
Also ancient woodland doesn't mean that there's actually trees growing there (let alone old trees) it's just a designation which means that it's not been used by humans for a given timeframe (granted a very long one - which is why it's protected).
Whilst it's easy to pick holes in any given plan, it's much harder to produce a plan which is better. As often by fixing one issue you create another.
There's also a balance to be had, do we look in detail at all the potential options or do we go with the best after a less detailed review? The UK always is said to take a long time to build something and it does the latter, how much worse would it be if we did the former?
That's not too say we don't review as we go to see if there's changes which could improve the design further (for example more tunnels to protect more landscape or creating wetlands to absorb more carbon).
Until COVID the numbers using rail were far higher than the predicted numbers used to justify HS2. Now whilst there's been a noticeable drop and a more reduced buildup to the previous rail use numbers - that's not entirely without government fiddling. For example insisting on reduced costs without understanding the impact of those changes.
Since COVID the Elizabeth Line had shown that people are still willing to use rail to travel, it just needs to be high capacity, run frequently and be affordable. If more of the network was like this (and we come back to the need for more lines) then it wouldn't be hard to see even higher rail usage than we had before.
It's unlikely that increasing ticket prices for the next (say) 3 years would have any meaningful impact on the level of support the railways required (as some reduction in ticket costs would likely result in some extra usage - how much each would be is unknown).
