Please please please let's not bring the FRA into transit.
As FTA regs stand, both "light" and "heavy" FTA-purchased rolling stock must "be able to withstand" collision with an FRA vehicle--however, interoperation is not allowed under any circumstances. Truly, the worst of both worlds. The US runs the most heavy and expensive LRVs in the world. The once-ubiquitous PCC car (revived in the 1990's by the Czech firm Škoda) does not meet this onerous (and arguably safety-detracting) standard.
(Remember, LRVs may operate in mixed traffic, meaning that any ped, bike, or auto is at risk of getting hit with that beast--and PCCs were deadly enough.)
There is a law of tonnage on our roads
but heavier does not always equal safer. Sometimes a meter or two less of stopping distance is worth a thousand crumple zones.
The Mattapan-Ashmont High Speed* Rail Line operates with aluminum body "wartime" PCCs and has multiple grade crossings. The T wanted to convert to buses but they were too heavy (!) for the elevated turnout at the end of the line. I have never heard of a serious accident on this line. Weight and safety are not the same thing!
*-It's a joke. Laugh.