Twinning is a more cost-effective way to have passenger rail separated from freight

Sure, but if you want actual high-speed passenger rail, the track needs to be straight and level.

The current freight tracks are as level as possible, but to achieve that, they follow the terrain, leading to some very twisty lines that require lower speeds.

Simply twinning those won't get near as much of an improvement for passenger rail as is needed.

It would be dumb to put in new lines and not make them high speed. We are so far behind the rest of the world it’s shocking. Not sure if you travel the world at all, but when you see what is available in other countries, you realize how embarrassing our country is in that regard. It would not be worth it to just put in more low speed rail lines, kinda pointless IMHO.