I suspect the primary reason for the movable frogs is that the back-to-back spacing of the streetcar wheels is wider than the mainline wheels. Thus standard switch frog and check rail dimensions for the mainline track would not provide the streetcars with adequate protection from picking frog points. Side benefit is no gaps which allows for smoother (and likely faster) running. Really a solid way to get around the problem, and presumably worth the added complexity. I wonder what interlock mechanisms exist to assure points and frogs are aligned; must be exceptionally reliable.

American interurban systems I believe solved this problem by making the streetcars comply with the regular freight wheel dimension standards.

What I have always been curious of is how places like San Fran worked back in the days of lots of streetcars with trolley profile wheels and street freight railways. There had to be times they were sharing switch-work. For a trackwork nerd walking around those streets would have been hours of fun study in track design.