How do you measure the externalities? In particular, how do you measure the environmental damage of air travel? Note that you must do it, as it is absolutely crucial for your entire argument. Otherwise, it might as well turn out that it is rail travel that’s more environmentally damaging, and we should tax it and promote air travel instead.

My suspicion is that you actually have not given any thought to this, have no idea about the actual cost of the externalities, and you don’t really care about that either. Instead, you just want to push your personal preference for trains over planes, while clothing this as if you were just appealing to objective facts and principles.

Government failures don't exist?

https://en.wikipedia.org/wiki/Government_failure

Let me introduce you to the concept of a strawman argument...