Is there a reason you couldn't build new light rail trains to a higher level of crashworthiness than they are currently? I don't know the full details, but that's how tram-trains in Sheffield, UK were allowed access to the main railway network.
Unfortunately no. The main difference is mass - US trains are vastly heavier than anything in the UK so by the time you make a tram crashworthy it isn't a tram any longer.
That said, I believe the FRA did allow lighter designs such as the Siemens FLIRT for commuter lines so the rules are definitely less onerous.
After a couple train crashes the FRA mandated PTC signalling everywhere, and in a world in which trains come to an automatic stop unless explicitly authorized to operate in the next segment, the old buff strength rules are not as important.
Also, the old buff strength rules were not great at keeping people alive. 25 people died in the Chatsworth train collision that led to the PTC mandate, which compares poorly to a similar crash between two trains in Germany which killed 12. There is a reason why buff strength has not been the criteria for automobiles for decades.
