Use train when you can. Trains are getting better and in Europe there is finally talk of prohibiting airlines on short-distance (<500km) flights.

But making trains faster and faster will not improve things as they'll also compete with other forms of transportation (cars, slower trains). If the faster train now gets you 800km in 3 hours, people will travel 800k where previously they spent that time traveling only 500km. It's Parkinson's law.

Maybe. I wonder which effect dominates if there's already a faster form of transportation that you're trying to replace though.