If the pressure drops below 900 mbar at the top speed you would be dizzy because you'd suffer from something akin to altitude sickness. "It's the statue, man, The Statue."
What I wonder is if the TGV is actually pressurized (keeping a inside pressure of 100 kPa) or just dampens pressure variations (more likely, because much less expensive) Un roi sans divertissement est un homme plein de misères
TGV lines have less tunnels, so making the trains airtight was less of a priority than for the German ICE. Still, though I'm not sure about what was built into the first generation and the TGV Atlantiques, from the TGV Réseau (the second subgroup of second-generation TGVs), they have a strong pressure isolation (for which is needed: double windows, isolation at car joints, and say pressurized air into the rubber bands around doors). *Lunatic*, n. One whose delusions are out of fashion.
As for limits on pressure changes, I found the following international railway norms: 1 sec: ≤0.5 kPa 3 sec: ≤ 0.8 kPa 10 sec: ≤ 1.0 kPa 60 sec: ≤ 2.0 kPa entire tunnel crossing: ≤ 10 kPa *Lunatic*, n. One whose delusions are out of fashion.
