SP:

Might be interesting to see how other trolley+freight systems work. VTA shares track with the freight branch to Moffett Field, for instance. And doesn't the Muni cross a freight line on 3rd street somewhere?

SacRT still has some single-track sections, mostly at the end of a couple of lines (so it's easy to allow just one train in there and make anybody else wait). RT also still has a single-track mainline section over a bridge that can't be practically widened, and probably uses signaling similar to what you describe for San Diego. When the system was built, there were several single-track sections, and I have heard that they had a wreck early on when a car ran through a red into single track with resulting low-speed (but enough) head-on. Rules & enforcement got changed ... as you and others have mentioned, rules are mostly written in blood.

BTW, San Diego chose to build the starter line without Federal money, not the other way around. By doing it themselves (local and state money) they could bypass a lot of contracting and design hassles to get it finished quickly and at relatively low cost (some would probably say cheaply). Inevitably, some headscratchers happened (like those poles you mentioned). Everything since then has had Federal $$, of course, so the newer (and rebuilt) stuff followed Federal standards and cost a Lot More to build.