I have never heard of any reason why an RDC would not be legal due to its original design. And remember - because they are self propelled they are in the same class as DMUs, and the FRA treats them as both locomotives and passenger cars.

The basic error was that Tri-Met did not initially purchase sufficient vehicles to operate their intended schedule, with spare equipment to allow for maintenance, breakdowns and repairs. My opinion - the Colorado Railcar DMUs are far too complicated than they need to be, but the current engineering consultant mantra is to add as many electronic components to things as you can, and the politicians buying them want something that looks futuristic.

Tr-Met did look at the WURR RDCs, but chose the Alaska ones instead.