The main difference is going to be scale and the chaos that brings. You see a LOT of streetcars in Toronto. For a huge chunk of downtown, it's the main subway-ish transit option. Some of the lines are on a 5min frequency, others are every ~10min or so, with some segments having 2-3 lines on it. I've seen them get backed up before an intersection, and open doors for passengers before they even reach the stop haha. I've personally hood-surfed over cars that got into the lane between the streetcar and the stop quite a few times, and sometimes it really isn't their fault.
The signal on the rear-end [1] is awful. There's a blurb saying "DO NOT PASS OPEN DOORS" which is clear enough, but there's also a sign that illuminates when the doors are open and it's NOT a stop sign. It's this do-not slashed circle with a picture of a stick figure seemingly walking into traffic. I think there's a reading of that symbol from well-meaning tourists that says "Attention boarding passengers, don't run into traffic". I could see that freaking out a waymo without a little bit of custom code.
[0] (All red lines are streetcars) https://cdn.ttc.ca/-/media/Project/TTC/DevProto/Images/Home/...
[1] https://blogto-production2-baselayer-display.blogto.com/arti...
