Maybe somewhat peripheral to this discussion but FRA is pretty firm on separatng anything lighter weight equipment from other standard railroad equipment. Examples include the MTA service in Baltimore, the NJT DMU service between Camden and Trenton, NJ and the new Escondido Sprinter. Any shared operations must have "temporal separation."

FRA has developed (and is developing) new crashworthiness standards for passenger equipment including corner post strength, emergency window/door exits, emergency signage, communication, etc.

This may seem a little heavy-handed but consider the Japan East Railway commuter train derailment in April 2005 (106 fatalities - caused by overspeed) or the German ICE train derailment in June 1998 (110 fatalities - caused by broken wheel). Both trains were built to a lighter standard than that in the US and it can be argued that the more robust equipment here has saved lives.