> a very large percentage of the rail lines are built either on elevated tracks near populated areas, or underground for sections

This is absolutely not true in France, where the TGV mostly takes regular lines to exit the suburbs of every city it goes through.

> It appears in the US many of these plans rely upon lower costs where the train is built on the ground. So they end up building tons of small bridges, regrading roads, moving wires, moving drainage, communication equipment etc as the train interacts with existing infrastructure

Building elevated is a shit-ton more expensive