FWIW, for many decades California has required crossing protection have three track circuits, two approaching (one on either side of the crossing) and an island circuit in the crossing itself.

There are also presence detectors, which use no track circuits, AFO crossing circuits (audio frequency overlay, "track circuits" without insulated joints), and predictors, which can be thought of as very refined versions of AFO circuits.

With coded track, which dates to before 1930, a series of DC pulses is sent down the track circuit, which is then decoded by apparatus at the signal end to display the appropriate signal indication. It was a very simple matter to change these DC coded circuits to AC coded circuits so the codes could be picked up inductively and decoded for display right in the locomotive.

Line wires: some ABS schemes use three line wires for signal control, others use two or four (this doesn't include power distribution wires nor communication circuits -- i.e. telephone and/or telegraph wires).

The polarity of a track circuit can also be put to use in place of line wires for determining whether to display a yellow or green signal.