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I agree a coast to coast high speed railway seems unlikely. But a Boston to Miami, for example, doesn't seem so hard. 500 km can take easily 2 hours for this kind of train, so I imagine Boston-NY would take even less than that.
High speed railways are always built from scratch. They have to, since they need to be much, much more straight than the traditional ones to avoid derailing. They are very expensive to build and requiere to expropriate a lot, so they are built by governments, at least around here. They started being luxury trains for passengers only, but nowadays there are low cost trains and freights too.
