Much of the French high speed rail network (LGV Atlantique,
LGV Rhône-Alpes, LGV Nord,
LGV Méditerranée,
LGV Est, etc) was inaugurated in the 1990s and 2000s.
This alone would have been responsible for a big increase in French rail traffic.
UK rail infrastructure certainly received improvements during this era too, but besides HS1 it was mostly just renewing and upgrading existing tracks. Nothing like the thousands of km of new high speed rail that was built in France!
If you just look at France it may be tempted to conclude it must be TGV, but that's the same methodological problem: it grew in most of Europe by comparable amount in the period.
Also TGV is nice, but it doesn't explain why TER and Intercité (the two slower-speed regional networks) also experienced an influx of passengers during the period.
It's not TGV, it's not privatization and it is not Wiedervereinigung, it's a broader trend.
> "Also TGV is nice, but it doesn't explain why TER and Intercité (the two slower-speed regional networks) also experienced an influx of passengers during the period."
I don't entirely disagree with you, but rail lines don't function in isolation. If you introduce a new high speed main line, there is a network effect: connecting services will also see a boost in traffic from people travelling on other lines to reach the new one.
This alone would have been responsible for a big increase in French rail traffic.
UK rail infrastructure certainly received improvements during this era too, but besides HS1 it was mostly just renewing and upgrading existing tracks. Nothing like the thousands of km of new high speed rail that was built in France!