I think there's a bit of complimentary too.. like you say, buses are easier to build up, which helps grow the market for shared transport between population centers. That's the market that needs to be big to justify rail investment. Also eg in Europe, they live together nicely with FlixBus providing frequent, fast and cheap service between cities but seemingly not killing the rail. Maybe this is an example of the Jevons Paradox.. it's actually the greater efficiency of each (over cars or air) that creates the greater demand