Interesting question. One that comes to mind is Boston's first picture-window rapid transit car that appeared on the East Boston Tunnel (now Blue) Line in the early 1950s. It led directly to Cleveland's new rapid transit cars a few years after and certainly influenced the design of the new air-conditioned Hudson Tube cars even later. All were made by St. Louis Car Company. An argument could also be made for the Chicago PCC el cars that preceded all of them, again all made by St. Louis Car.
Cleveland's introduction of the very long Airporter cars in the late 1960s might have inspired subsequent longer designs, but who knows?
On the light rail side, however, quite a different story obviously- from the PCCs of the 1930s/40s/50s to the Siemens-Duwag cars of the 1980s and 90s. "Dare to be similar" was clearly the philosophy, although UMTA did force a joint LRV procurement from Boeing for Boston and San Francisco.