The PCC was /is an example of properly conducted real-world based research. The fact that few ,if any, further advances in design have come to light proves that. Boeing-Vertol tried to re-invent the wheel IMHO and failed. The Siemens, Bredas and others are companies that have been in the business of building rail vehicles for a long time and are using time tested technology. The reason that their equipment looks the way it does is that a major portion of the design is geared toward ease of maintenance. Outward appearance is secondary to safety features and ease of operation.
Streamlined steam went through the same process , maintenance forces,piece by piece,removed and discarded streamline parts that added down time to the service cycle. The European thinking is more toward modular design. Take the bad module out ,replace it with a good module and repair the bad one while the car stays in service, maybe even while the car IS in service. With telemetry a bad order car could tell the maintenance dept. what is malfunctioning and where the car is on the line. A maintainer can meet the train with the replacement module and swap it between stops. Virtually zero down time.

On another note, The thing that bothers me with modern light rail operations is the adoption of fixed stations as the ONLY way to get on or off the train. When the train passes where I need to be to drop me at a station 6 or more blocks away, I am more likely to drive my car than take the train. What happened to the car stop at the corner on request. The Sacramento system has some enormous gaps between stops. Transferring from train to bus to go to a place that the train passed is maddening.