So France isn't serious about building a meaningful HS network? They didn't even tweak it to make TGV Nord and TGV Est terminate in the same station, but instead two near each other...
If people are expected to make cross-Paris Intercity journeys by carting their luggage across on the Paris Metro, then yes, France isn't doing it properly either.
As for excuses, how about:
1)a single London terminus for High Speed trains to all over the country (including the South Coast as well as Kent, Europe, North and West) will feed too much traffic onto the tube network for adequate dispersal of traffic through central London (even if each line had their own OOC-style 'get a third of the traffic off' station).
2)a single London terminus for High Speed trains would have too many platforms.
3)security checks for the Channel Tunnel puts paid to easily and practically having a through station with through running and no/few terminal platforms.
4)the geography and demand patterns likewise prevent that sort of set up
5)huge expense for little gain
1) I'll concede that point, but I don't think it's insoluble, it would just require some serious thought about how and where to put tube entrances. Paddington has two separate tube stations, a similar arrangement might work for an HS terminal.
2) How do you justify "too many" platforms? Grand Central, NY has 44. Is that "too many"?
3) That depends on whether we're talking domestic or international HS travel. I'm largely looking at domestic travel here.
4/5) On what do you base these claims?
Sure, 45 minute journeys on the tube (is there even one inter-terminus journey in London that takes that long?) is not good, but a couple of minutes on the tube, or a 10 minute walk, or a 5 minute travelator journey isn't really a hardship - the latter two transfer times could well exist in the same station (and certainly do in airports) - especially if the station had most of the capital's intercity trains serve it!As long as 19 minutes? Heathrow Express takes 15 minutes between Paddington and Central today. Take 4 minutes off for Paddington-OOC, and wack it back on for being on the slows and stopping at Ealing Broadway and Hayes & Harlington if (when) HEx is taken over by Crossrail. That gives 15 minutes.
Last time I checked the National Rail journey planner,
every inter-terminus tube journey takes 45+ minutes. I've had tickets issued with up to 70 minutes to make a connection between Euston and Waterloo.
Connection times in airports are less of a fair comparison, as you don't have to cart your hold luggage between terminals.
Anyway, all that said, I realise that it's not so much about requiring a single terminus station, but more about removing the need to cart heavy bags on to the tube, or for the NR journey planner to have to build in whacking great amounts of contingency time. Solutions to that, such as a travelator between stations are perfectly acceptable as a compromise.