Would Republicans support a major spending bill to build a true high speed rail network in the US?

No, but the party is irrelevant for that spending bill could be ped by reconciliation and put in the budget. They are the party of no, and no to technology and advancement as we have a 70 year old crumbling infrastructure but we have lots of nucs and great airplanes to bomb defenseless people from 70,000 feet. That is what they would vote for!