[choeger]

This. People tend to grossly underestimate the expense and effort to build new rail tracks in an industrialized country. Most land is owned by someone already, there are environmental concerns, other (rail-)roads and generally obstacles must be crossed, and -worst of all- you have to repeat the process basically for every single town you want to connect.

Side remark: The same holds for roads, of course, with the notable caveat that roads form a flexible network automatically. Planes are just superlinear in their flexibility: If you have n airports and add one more, you get n new connections.