> What is holding US back in public transport?

Various things, surely, but things like property rights and an unwillingness to accept wholesale use of eminent domain. A 150mph train requires shallow curves and therefore won't run on an existing freight right-of-way. If not for that, it would be pretty straightforward to build.

Meanwhile, a plane ride from Portland to Seattle is 30 minutes and cheaper anyway. There's no incentive to invest in high speed rail.