>On Santa Fe's old 4th District TCS to San Diego, the advance approach (flashing yellow) was done through another set of line wires.

This is the normal way. I wish now that I had asked that signal maintainer how they got the 4th aspect in the Suisun marshes because there weren't any line wires.

>On a straight battery circuit grade crossing (approach-island-approach), Santa Fe had a timer on the stick circuit to drop it out after 10 minutes. Southern Pacific apparently did not use a timer, so you could tie up a train on the far side of a crossing while still in the approach circuit. But this also allowed the stick to stay up if the track circuit failed as the train was leaving, and the next train from the opposite direction would hit a locked out circuit and not drop the XR to trigger the crossing until it hit the island - which I witnessed once. (A potential disaster but fortunately no cars or log/gravel trucks at the crossing.)

The approach-island-approach scheme has been required for several decades, at least in California, with simple two-coil interlocking relays no longer permitted either. There may have been grandfather clause provisions for existing installations when this prohibition was established. What you describe sounds like the trap circuit used on diamonds.

>The SD&AE had some interesting circuit before the trolley. One had three approach circuits on each side of the island. The first was a timing circuit.

In territories with high track speeds and busy crossings, the SP used a series of timing circuits.

>Most of the light rail system's technology and operating practices differ entirely from typical heavy rail practice. My previous mention of the San Diego Trolley and Sprinter was because freights do use part of the same system.

In many cases, transit operations use(d) similar signal systems and apparatus, case in point: the Bridge Railway (Key System). Except for some relatively minor details, it was virtually identical to what the NEC used between 1930 and 1990. However, it's ill advised for our readers to assume transit signalling = RR signalling.