If the chief argument for building BML2 is that it gives Brighton access to London if the current mainline is shut, why not build a north-east curve at the Arundel Jns and route services via Worthing, Horsham then either Gatwick Airport or Epsom if need be? Would take longer but it'd be a damn sight cheaper than £315,000,000!
...a lot longer. I've done the Arun Valley tour, lovely in daylight but interminable after dark.
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"Japan is one of the few countries in the world where construction costs even more than it does in western europe, so the costs should transfer relatively well.
Brighton-London is only ~75km so we could either cut the price roughly 40%, or extend the line along the coast all the way to somewhere in the vicinity of Portsmouth.
As improved service to Portsmouth is not included in the BML plan I will chose the former.
This gives us a price for the project of roughly ~£2.4bn."
Looking at the furore surrounding the HS/2 route, I can only imagine the outcry at a total new build in southern England. One of the key points in favour of BML2 is that, other than Ashcombe (SW of Lewes), it utilises existing or moribund infrastructure as far north as the proposed dive underground just south of Lewisham. The tunnelling costs indicated on the BML2 website, I know, are based on figures for HS/1, and whilst I can't comment on tunnelling specifications, or the geology in Japan, an earlier proposal to tunnel into the centre of London from Coulsdon doesn't bear out tunnelling being a cheap option as you suggest - neither does the Bakerloo extension scheme, so I can see no valid basis for comparing the Japanese case with the UK. A bore though the South Downs would be in chalk, marle, gravels and clay. The northern tunnel would present broadly the same geology as the DLR extension.
Insofar as BML2 does nothing for Portsmouth, I wouldn't disagree, but would say I think it's rather beside the point in a scheme designed to address capacity problems from East Sussex, Kent and East Anglia (which equally, any scheme designed to meet requirements in Hampshire couldn't reasonably be expected to achieve). There's also an issue with congestion on the cross country A27/A26 route between Brighton and Tunbridge Wells, a route which becomes possible with BML2. Another point overlooked is the assumption that only the towns I've already mentioned could benefit from the scheme. The number of paths potentially released towards London would allow improved access from the West Coastway line.
I see there is discussion of improvements in the SW London rail network, and would suggest any improvements on the Hampshire lines would be better treated in conjunction with these. BML2 can't pretend to be a panacea for all shortcomings south of the Thames, but with Docklands and now Olympic Legacy redevelopments, the centre of gravity in London has shifted eastwards, and lack of infrastructure here
is addressed by the scheme.
On the question of electrical supply, I confess to looking at things differently from the BML2 project. I view the adoption of OHLE across the old Southern as inevitable and as BML2 links with 25Kvac lines north of the Thames, and bearing in mind that the line via Gatwick would eventually be converted, see the project as able to mitigate the inevitable chaos of conversion. I'm no S&T expert, so I can't comment on the difficulties this would cause, or how they would be overcome.
Your £2.4bln estimate (cheaper by far than figures being bandied about for the Barkerloo extension - and the Hayes branch would do better as part of the BML2 scheme) I haven't the specific information to confirm or deny. Until the scheme warrants full costing, we're all just estimating and HS/1 aside, I'm not aware of many estimates for capital schemes than have been particularly accurate - especially in the rail industry!
What I (and every commuter from the south coast) do know is that the present line is running at capacity for large parts of the day. NR identify key bottlenecks which will have to be addressed. I can't see the Japanese (or French) approach being politically possible here, so new formation is out of the question. Quadrupling the Brighton line south of Three Bridges, leaving cost aside, does nothing to solve problems at East Croydon and nothing to lessen the vulnerability of the route overall. Any solution to capacity issues at existing London terminals does nothing to improve services to the regenerated east end. No other scheme I'm aware of answers the fundamental capacity issues raised by NR, DfT (or Gatwick Airport's management) concerning rail capacity.
True, it's not a cheap scheme, neither is the work at Reading nor is Crossrail. BML2 is a major infrastructure investment with major benefits in the most populous and congested corner of the country. I recall similar arguments against Thameslink, and that's the sort of project BML2 needs to be compared with.