Just taking a simple example like Lausanne (Switzerland) to Paris (France)¹ shows comparable ticket prices. For an arbitrary date a few months away like April 21st, that gives me a single train ticket for €65 on a direct connection to Gare du Nord, or a train ride to Genève-Aéroport (€8) plus a plane ticket for around €65 to Orly. Cheaper plane tickets exist, but not if you have even a single backpack.

In terms of time, the train is a direct TGV that takes 3¾ hours, while the plane route means taking a train for ¾ hours, waiting at the airport for at least an hour because of security, flying for 1¼ hour, wait for another half hour at least to get out of the airport, and then spend at least half an hour getting to where you actually want to be in Paris. It takes just as long, but probably longer if you want a decent safety margin on the airport.

And that excludes the simple fact that we all have a responsibility in reducing our carbon footprint, and that for most us who have the means to travel taking a route that may be (but isn't in this case) slightly more expensive and take slightly longer is the least we can do if it is the greener alternative by far.

1: Just an example of a journey I took myself, by train.