The only way mega-cities like London will be able to develop sensibly in the future is by becoming more polycentric, as Tokyo has. Simply increasing the density of the existing dense core ever further is not going to be sustainable, in transport and other terms. This process started in London with Docklands, but that doesn't have a long-distance mainline directly going through it, so connections had to be developed instead. Stratford is part of that result, which clearly IS on a mainline, and TOCs aspire to stop more trains there; it is starting to become a major centre in its own right too. It makes sense to develop future centres at similar transport hubs on mainlines around London. As they grow, they will naturally attract new local public transport networks, which can also serve a long-distance terminal, if the hubs are so equipped, to act as distributors/collectors for the services calling at that terminal. I tend to think of GWR long-distance stopping at OOC as SHARING HS2s local connectional network in a similar way to Kings Cross and St Pancras sharing their tube and bus connections today, rather than primarily facilitating the connections between the long-distance networks themselves (although there will inevitably be some people doing this). While no one has yet mentioned it, I feel also that OOC will be a far more convenient drop-off and pickup point for many people being driven to or from the station than Paddington, or even wanting to park their own car at the station, although I don't know what parking provision is intended. Certainly, the quantity of brownfield land in the area could easily support some significant parking provision if desired. This could serve to greatly increase the attractivenes of both HS2 and GWR long-distance services for the many west London residents who might be put off by a backtrack in the wrong direction all the way to Paddington or Euston first. A West London Parkway perhaps (although I'd never call it that personally, mainly because it will be far more than that anyway!).