Hi
These were introduced in Tane and they are used by some assets to indicate when a track is occupied. The track insulators are used to mark the limits of the track section - you may need more than two if there are crossings or junctions that branch off of the section that you want to monitor. The track Circuit Detector is the bit that checks if the section is occupied and must have a unique name.
I use them in conjunction with the Enhanced Interlocking Towers by author "pguy" to find empty tracks in stations or yards or to control single line sections to prevent cornfield meets. When used with the same authors Mission Codes it is possible to run long sections of a route with just a couple of commands. I know you can do the same with Drive/Navigate to/via Trackmark but using the Enhanced Interlocking Towers mean that route setting is taken out of the hands of the AI so the session works smoothly without pauses.
Pierre Guy has a small demo route on the DLS to illustrate the use of the Enhanced Interlocking Towers and how the Track Circuit assets work with them. It only downloads a handful of assets so it is worth having a look at it if you are interested.
"EIT DemoRoute - UK 1", author "pguy", kuid2:61392:8401:6
I don't know of any other assets that make use of the track circuits but hopefully someone will chip in if they know of any others. If you are interested in automating sessions for the AI then it is well worth spending the time to understand Enhanced Interlocking Towers, Mission Codes and Track Circuit Blocks.
Regards
Brian