The transport industry view is that when the high speed rail time is under 4 hours you start the modal shift, but it really kicks in under 3 hours. Under 2 hours and it’s both largely game over for flying and rail creates its own self sustaining market. Look at Paris-Lyon post TGV.

My last Athens-Thessaloniki train trip was in 1993 in a battered couchette car hauled by a spectacularly loud ex-DB V200 loco. A great experience in some ways but perhaps one left in the last millennium 😉