If you have ten airports, you have 10 * (10 + 1) / 2 = 55 airlines. If you have 10 train stations, you have only as many rail lines as you're willing to build, which means countries end up spending most of the effort and money on lines connecting major city centers.
Also, if you have an international airport, you're connected to every capital city of the world.
The logistic advantages of air travel are huge. Kerosene will need to get a lot more expensive before these advantages lose out.
Also, trains can switch lines, disconnect, run on the same line back to back etc.
Just look at the Europe railways map: https://commons.wikimedia.org/wiki/File:High_Speed_Railroad_...
I suspect, with some standardisation and automation, you can end up having very good point-to-point travel options by train. Even better than airlines maybe. The current inefficiencies could be due to poor management and weak integrations. Can you imagine how great could be to have Europe and Asia wide rail network where automated carriages and rail switching systems coordinate to achieve travel from any train station to any train station?
Airports also tend to be at the middle of nowhere, IMHO you need to incorporate the cost of building and maintaining the infrastructure needed to get somewhere from that nowhere.