Originally Posted by BAAZ

No. It isn't 5 hours door to door, it's 5 hours St Pancras to Frankfurt Hbf. Experience elsewhere in Europe suggests that high-speed trains start to compete effectively with air travel around the 3-hour-train-journey mark. Much less than that and the train is the clear winner; much more than that and most people prefer the plane.

It would be very similar to London-Edinburgh, where most of us would probably choose to fly.

Now, an overnight sleeper train, that would be something else entirely. Leave London at 11pm after a good dinner, travel on the new and smooth high-speed track without all those jolts and bumps you used to get, with the train perhaps slowing down from 300km/h to 200km/h for a few hours during the night when that wouldn't cause an obstruction, in order to wake up in Frankfurt at 7am. That would be really tempting.