Look at the swedish servergame on the line between "Falun Station" and "Dingtuna North Station" which features three aspect track circuit block signals.
What happens is this:
* A train traveling the line reserve a number of signals ahead.
* All signals in the reservation shows CLEAR (green and white), including the last signal before the next red light, which should display CAUTION (two greens)
* When a train approaches a signal, the state of that signal will change state to CAUTION!
In order to see the states of the signals better, especially the ones from behind, dim down the daylight in the game.