France has roughly the same population as the UK but around double the land mass with far smaller city populations outside Paris.
That said, there are now plans to reopen the Val-de-Grace Military Hospital in central Paris that was closed a few years back as surplus to requirements.
Presumably converting a couple of trains to access existing hospitals with spare capacity is quicker than re-activating a mothballed hospital. If the infection spreads to become equally prevalent across all regions then all existing hospitals are likely to be roughly equally under pressure so there would be little point in shuttling people between them. However the trains could then have a role in bringing people to Val-de-Grace, which I see is close to Austerlitz and Montparnasse and therefore conveniently accessible by TGV from much of the country.
The TGVs transport people in a stable condition but needing ICUs from Alsace and Paris to Aquitaine and Brittany. On the other hand, VDG is likely to be providing pure bed capacity for medium symptom individuals in Paris as these people can't be transported en masse to elsewhere.