I think you might have said that earlier in your post

The problem is that your proposed solution is not a one sized fits all solution. Many rail questions do not have an obvious home board, like the Shinkansen in Japan do. In Europe we have trans-border trains. Should Eurostar go into London, Paris, UK & Ireland or Western Europe? That's the issue we have in the UK - I've seen UK rail related questions over five different boards (UK & Ireland, London, BD, BA, Travelbuzz, and i'm not a VS reader so I don't know if they turn up there also). Unfortunately, you haven't really read this thread - the BD regulars have not objected to it being used for UK rail questions as well, since many of us use trains and planes interchangeably for domestic, or short-haul European travel. But at the same time it's a clumsy patch, and people outside of the UK-based boards tend not to be aware of the convention, because it's been an unoffical ad-hoc arrangement.

If there is a logical place for rail questions to go, then by all means, post them there - I wouldn't expect to see London tube questions for example, in a generic rail forum, but to continue to be posted in the London board. But it gives a home to those questions where the place to post is not so obvious - Eurostar, cross-border trains etc.