While I had no idea there were so many variations on the design, I'm inclined to believe that the differences which separate today's light rail lines would be far more challenging to overcome than many of the differences which caused such a wide offering of PCC designs.
With few exceptions, a streetcar line is a streetcar line. All operate under similar conditions (tight corners, traffic, need for high acceleration, need for strong brakes etc.). Light rail lines now are constructed to a standard that generally reflects heavy transit of earlier years (massive concrete structures, private ROW etc.). Because of this massive infrastructure, modifying it to match a standard railcar would be much more difficult than I imagine it might have been to modify a line for PCC use. This massive infrastructure, in the form of bridges especially, is almost always built to handle very heavy railcars, some weighing 30 or more tons. To build a standard railcar today, one would have to ensure that either the railcar was light enough to meet the weight standards of bridges on the least heavily constructed line or upgrade bridges while ensuring future bridges are built to that standard. I'm not disagreeing with anything anyone is saying here, by any means. I think we all know any such modern "PCC" would require huge changes in both the ways we think about and the ways we build light rail.
Of course there was the difference in voltage, but generally that was a matter of volts DC. I would imagine that, due to the technology of the day, most systems were 600 volts DC or 750 volts DC. Today we have DC lines of 600/750/1200 volts and AC lines of many differing voltages. Any "standard" car would have to have many different offerings in the form of electrical equipment.
Accessibility is another place where a "standard" would fall far short of being "standard". Some systems require low floors to load from street level, others require wheelchair lifts to load from curbside/street level, still others require high level platform loading, and most require a mix of a few loading options. Even leaving aside the just and fair requirements of ADA (whose existence is maligned by the ignorant every day), this variation in accessibility would cause many problems for anyone attempting to realize savings from a standard design. I suppose someone might be able to find a way to allow all such methods to be allowed through one design, but I'll leave that for someone whose intelligence exceeds my own. Throw into that design the ADA's requirements and you are looking at one hell of a complex system to protect all options.
Again, I would think it would be much easier to approach this from the infrastructure end. National governing authority not being the "in" thing with many voters right now (most of whom really like the benefits of their interstate highways), I can't imagine anyone making an attempt any time soon to federalize standards for light rail line construction. It's either that approach or we look for cheaper methods to allow the return of streetcar service for which a PCC design might be offered. Streetcars are not about to make any real comeback until we find a way to address the ridiculous price-tag per mile. The 7 some odd miles of my favorite, long gone line (29-Bloomfield) would be nearly a billion dollars if I was to build it using the prices of other lines in my home state.
Be well. Do good work.
Semperfidelis