Why should we be so parochial in our tax / spend? We’re not the US. It has a decent regional and network benefit in the SE. It’s not really so ambitious - 12 months and 80m in Madrid.Who should borrow and cover the cost is the key issue. Why the heck should London council taxpayers subsidise Herts?
Methinks that your views about costs and revenue are hopelessly optimistic too - funding wouldn't have been an issue if this was a potential goldmine.
Paris also opened their minds beyond the arrondissements, and 14+ are thriving without objections. Why must we be so small minded?
Watford Junction could again be a major railhead for outer north London, that’s worth something. Plenty of jobs there too, bleak as it is.