I'm not sure that an 8 hour route between these cities with ~900 km between them really makes our case here. I don't think there's really a strong argument to be made that taking the train is better than flying between these cities.

International trains in Europe are a weak point of our network, one which we desperately need to improve.

That no one travels from Germany to France is of course entirely false. Frankfurt to Paris would be a far stronger example, coming in at 3 hours 30 minutes for ~500 km.