OK - I'll take a stab at this one. "Heavy rail" is built to meet FRA or Transport Canada standards, so as such both can operate together without restriction except when it became an issue on the Northeast Corridor. "Light rail", regardless of equipment weight, does not meet FRA standards, and is instead governed by the state's PUC (or equivalent agency). I think there are some federal standards and an industry group involved.

There are combined systems, such as the San Diego Trolley and Sprinter where both Light Rail and freight use the same tracks. Originally, the SDIY/RailTex would wait until the trolley dropped to 30 minute headways (10 pm?) and then run between them from San Diego to San Ysidro, and return before the morning rush.

As I recall, there was a accident somewhere in the east where a freight and an LRV train tangled. They were on adjacent tracks on a shared right of way, one derailed and the other side swiped. Obviously the laws of physics apply when masses of substantial differences collide. Soon after that the FRA came out with a new interpretation as to what rules apply. I don't know of any restriction on shared right of way, except that any heavy rail and light rail systems must now be separated by at least 30(?) foot track centers for the light rail to be considered "insular".

In your examples, WES is heavy rail. UTA built a parallel track on the same right of way for heavy rail, and the Salt Lake trolley is light rail only.

FRA threatened the MTDB to bring the trolley under FRA jurisdiction - just the cost of changing the glazing to FRA was going to be huge. The compromise that both the SD Trolley and now the Sprinter operate under is separation by time - only freight or transit operations, never at the same time. The light rail operations and their equipment are under Cal PUC jurisdiction, as far as I know the track and signals used for freight and their operations are under FRA jurisdiction and standards. My guess is that any track and signals used solely for light rail operations more than 30 feet from any heavy rail operations are under PUC.

As far as Santee, the SD&A/SD&AE used to go through Santee to a station called Foster. This flooded many, many years ago and the line was abandoned beyond El Cajon. Any of the new trolley lines built beyond the SD&AE do not have freight operations and are only built to light rail standards. This includes the street running downtown, the line to Old Town and the stadium that connects at La Mesa, and the line between El Cajon and Santee. It took SDIY two nights to run a train from San Diego to El Cajon to switch the few remaining industries there the last I knew. Pacific Sun runs at night to switch the few industries left in Escondido, most of the line's feed mills were bribed/forced/conned into working out of the Perris branch. I think the old EVPA mill in Escondido is the last major customer left. Pacific Sun also runs down to Miramar to switch the few remaining customer there. At one time, the Escondido Local worked five trips a week from Oceanside plus switched the yard on Saturdays, the Fallbrook local also ran out of Oceanside until it was abandoned, and the Fourth District Local ran out of San Diego five nights a week to Miramar, Oceanside and as far as San Juan Capistrano if needed.

There are a very different considerations between heavy and light rail. Light Rail Vehicles are capable of much shorter stopping distances, especially if they have magnetic track brakes for emergencies. With street running, their signals have to be different so vehicle traffic isn't confused. Most US systems have been forced to dedicate road lanes for light rail because US drivers just can't fathom the idea of yielding to rail. I think most of the today's collisions are drivers making illegal left turns in front of a trolley. The track circuits themselves have to be much more sensitive for most transit versus railroad applications due to the lighter vehicles and shorter trains. Most US railroads have rules that require a dispatcher to provide additional protection for a Sperry car, smaller light engine move or engine with just a few cars because they may not shunt the track circuit on a consistent basis. Stopping one of them on a heavy layer of sand can actually insulate the wheels from the rail.

I know transit is very different than heavy rail, using different braking and signal systems, and operating rules. I have very limited knowledge and no experience with transit. It is hard enough to keep up with heavy rail.