Looking closely cars (some at least) have a U-shaped panel here. Presumably there's something that must be moved for towing ...
Good eyes! Now I'm going to have to keep my eyes open on the street!
Looking through the photos at https://cptdb.ca/wiki/index.php/Toronto_Transit_Commission_4400-4603 I don't start seeing it until 4430, after that its consistent. 4427 doesn't seem to show it, and there's no photos posted there for 4428 or 4429.