The crashworthiness directives from the FRA require the tank like build of the locomotives and various cars.
I work in the Aviation industry, and trust me. The FAA does the samething to aircraft. You'd be suprised how much lighter aircraft could be (and thus save weight) if it weren't for environmental issues, excessive stress margin redundancies, etc.
The biggest problem in the US passenger train world is that, quite frankly, we're not good enough at preventing accidents to begin with - whether signalling, grade crossings, driver error, etc.
One reason the Shinkansen hasn't had a single fatal accident (other than suicide or natural causes) in its 50 year history is because of state of the art signalling, isolated right of way (ONLY Shinkansen trains run on Shinkansen tracks), and no grade crossings.
The latter two have been compromised a wee bit with some of their new routes that go out Northwest from Tokyo like the Nagano line, with accompanying accidents. My reference is to the main Tokkaido line, although the Joetsu and Tohoku lines are built to pretty much the same standard as the Tokkaido.
Anyway, prevent accidents - or significantly reduce the probability of one - and the FRA may be convinced to reduce the crashworthiness standards.
Incidently, the FRA is also pretty dumb. Those famous Bombardier Bi-Level trains? They have some club seats with tables. In one of the Metrolink crashes, that was the cause of the only fatalities on board - blunt force to the gut. Passengers moved, but the tables didn't. NTSB made a recommendation to remove the tables or let them break away. But to my knowledge, they still exist and they are still dangerous.
Even in any Amtrak train, I prefer to sit at a table facing backward for that very reason.
The onboard labor is the big issue for the long distance trains, not the fuel. A 20% savings in fuel & power is only 28 million dollars in savings. Where the built like a tank thing comes into play is really with acceleration, or rather, the lack thereof.
I think it's more in the retirement pensions and station personell. You gotta have people onboard the trains. I mean seriously, some of the full service stations that serve three trains a week yet are open 72 hours a week is ridiculous.