Nice to see another option to get from Athens to Thessaloniki. Does Aegean codeshare on any train routes historically?

At very high speeds, the view out the train windows aren't as easy to appreciate as the view out the windows of trains running at more common speeds, but I would assume that for Athens to Thessaloniki there will be some significant stretches where the train is going well below the maximum cruising speed.

Are we to expect free internet on board this high speed service?

The actual flight time between LGA and DCA can at times be around the same 30 minute flight time as ATH-SKG. NYC-Washington has the Acela train service which for some limited stretch can reach up to 240km/hour, but mostly the journey takes around 3 hours by train since it can't maintain that speed for even a quarter of the journey. Most of my acquaintances whom used to be frequent flyers with me on the shuttle flights between Washington and NYC have mostly jumped onto taking the trains in one or both directions since quite some time back. If your business involves going from city center to city center, a 3-4 hour train trip can be better than dealing with airports and flights even when there is an actual flight time of 30 minutes as an alternative to the train.