IOW, build another railroad. UP would love to have it double track as it is, but there are too many places where that just won't fit. So another main track would have to be inside a mountain or hung off the other side a bit precariously. By the time you're done with a complete 2nd main track, you could have built 1/2 of the HSR line, on a better alignment for passenger (rather than 23 mph speed limit).