Where I depart from your earlier posts is the impression that major upgrades would be needed for an hourly Bedford to Oxford train, running at the current line speed, prior to the Cambridge bit being built.
The point here is: it’s best to do the mitigation once at each location, rather than, say, upgrading a footpath with miniature stops lights for the extra train an hour, and then having to come back and build a bridge when the full service to Cambridge starts. Also, there
will be crossings where the extra train an hour - which is a doubling of the service, let’s not forget - drives change. There’s plenty of examples of lower increase driving chance - Redhill to Guildford, East Suffolk Line, Ely-Peterborough, Cambrian, and more. What can be done in these scenarios is take a route wide approach, and spend the money where it best delivers safety benefit, rather than at every crossing. A hypothetical example - if a route has 10 crossings, and the risk increases by 20%, then rather than spending £2m to reduce risk at every all 20 crossings by 20%, it might be better to spend £1m by closing 3 of them (if that’s possible).
I agree that such a risk assessment might throw up the need to stop up or replace some foot and perhaps occupation crossings with bridges, however that is a fraction of the work needed to replace the road crossings with bridges and shouldn't, I would have thought require a TWA to do.
Yes - closing or diverting any public right of way that crosses the railway usually requires an Order under the TWA process, or similar. Same for private crossings unless the authorised user agrees. Hence the TWAOs across Anglia (the Cambridgeshire done has just been made), and the same for the WCML upgrade. Replacing a User Worked Crossing (vehicular) with a bridge would indeed be a fraction of a public highway crossing - about 8/10ths. A bridge for vehicles is still a bridge. The usual method of closure for UWCs is to reach a settlement with the authorised user and build a fence. Often this involves buying the field(s) that the crossing gives sole access to, then promptly selling them to the next door farmer at about the same price. Still costs a fair bit in legal fees, compensation etc.
In this case, the TWAO was for the whole route from Bicester to Bedford, and includes the stopping up of a number of crossings, often without providing s bridge. The Inpsectors report is worth a read to see how this was determined. It’s nearly 400 pages. As far as I can see, there’s no road bridges being built between Bedford and Bletchley, although they were argued for by the local authorities.
The ratchet tightens ever more all the time though and "reasonably practicable" gets ever nearer to "remotely conceivable"
As I mentioned earlier, the latest (draft) guidance from the ORR does explain what should be considered reasonably practical. In my view it’s about right. This is useful, as 10 years ago the definition of ‘reasonably’ varied from Inspector to Inspector, leading to all manner of delay, unexpected costs and ‘robust discussion’.