I've seen power-related issues with Sacramento RT, especially during very hot weather. Trains slow down, and run fewer cars, to conserve power. The older (1980s) cars work better than the newer (2000s, with better a/c) under those conditions; they don't have total computer control, so aren't as dependent on having full voltage available.

Putting on curmudgeon hat: then there are old-fashioned streetcars; I used to be on PCCs in SF where voltage dropped to 300 (from the nominal 600) or even less downtown at peak times, but the cars kept moving, slowly, with dim lights. Modern cars would simply refuse to move under those conditions.