The Y shaped network that has to gather all the north-south traffic from all three existing corridors and funnel it into a single pair of tracks in order to fill them.
Which produces a railway with operational characteristics seen nowhere else that I am aware of. There is no analogue to the London-Birmingham HS2 route anywhere that I know of.
It's (in that original context) quite like the LGV Paris-Lyon (apart from the higher frequencies): half-a-dozen regular services of which only one actually serves Lyon, the rest bypass it.
Looking at the LGVs overall (and ignoring all the politics), the intermediate stations are a mixed bag:
Sud-Est has a couple of intermediate stations with 2-hourly services to Paris - pretty minimal in terms of line occupancy.
Nord has the original "gare betterave" at Haute Picardie which was intended for Paris traffic (and failed!) but has come into its own since the Interconnexion opened (I wonder if it has become a remote car park for CDG?)
Est has Reims-CA and Meuse, both predominantly served by Province-Province trains, ditto Massy (which be a French Stevenage if SNCF were interested).
Atlantique has Vendome which is clearly Paris-orientated, Bretagne has nothing on the core section.
Anyway, I too have often wondered whether a Bucks HS2 station should be provided: the French evidence seems to be that it would only work for London traffic, rather than for (eg) Aylesbury-Manchester.
So if it turned out that there was
long-term spare capacity on HS2 (which might be the case with nothing going to the NE) and if you agreed to build a Buckinghamshire New Town by the line, you would then add a station and run very fast commuter trains to Euston (say 4tph like Ashford); perhaps if there were still a spare path you might add 1tph to the north.
That's two big ifs - and the second is vanishingly unlikely IMO, certainly in the next 20 years.
