If you look on Amadeus (and other GDS's), a number of AF flights start or finish on the train already. For example, a flight from Philadelphia to Brussells via JFK and Paris will use two railway journeys that carry AF flight codes.
A good idea because you don't have to wait ages in, or change, terminals in many cases.
It also protects valuable slots at the major hubs. If we could finally get our main lines sorted out (and rail fares!), why not have MAN or LBA to LHR as a rail service.