I like the idea of some sort of rapid transport connection between Euston, St Pancras International and Domestic and King's Cross, but further underground passages, tunnels, etc, seem too complicated with the existing services already there, and the British Library in the middle. But if anyone had a few billion pounds spare, what would be the problem with a monorail system, rather like that at Birmingham Airport, linking the three? With lifts/travellators from ground level, it could be an easy way to make the journey with luggage, and might even rival the London Eye as a tourist attraction in the process!