It is very strange that countries like Austria, Japan or Switzerland have some of the best rail systems in the world even though their bridge and tunnel requirements are huge. In the US building rail on any terrain seems to be more expensive than basically anything one can build in Austria.
Here is a huge/difficult island with only 55K that has tons of tunnels: https://maps.app.goo.gl/abcnXtRNs9QdVkhC8
Population size, density, terrain, etc. have nothing to do with it.
Not strange at all! If you want to go by car you must build even more tunnels. Mountainous regions favor rail just like urban areas do. Furthermore, 19th century investments into rail still pay off in mountainous regions, because once you build a railway bridge or tunnel, you are kind of dumb not to use it. In the USA competition from trucks or cars is much tougher.