Taxing? Stopping to subsidize them is already a good idea. Also: climate goals and airtraffic don't mix well, so a regulators goal should definitly be that the most climate-friendly ways to travel are also the easiest, cheapest and most comfortable and reliable.

As a European who has taken more than one 16 hour train ride where I could have taken a 2.5 hour plane trip, I really think night trains done properly could be a game changer.