TL;DRIn the upcoming version 0.15.11 a station is able to send a unique Train-ID to the circuit network. I suggest that track signals should have the same feature.
What ?According to this post by a Factorio dev, version 0.15.11 will introduce a new feature that stations with a train stopped at it will be able to send the unique ID of the train to the circuit network.
I suggest that track signals should have a similar functionality. In "pulse mode", whenever a train passes that signal, they pulse the Train-ID to the circuit network. In "hold mode" they send the Train-ID as a continuous signal to the circuit network, as long as the train occupies the block after the signal.
EDIT: As stated in a later post of mine, I believe it would also be useful if the track signal would also be able to send the cargo contents of the train to the circuit network, instead of only the Train-ID.
Why ?This would allow the circuit network to keep track of the position of individual trains.
Related suggestionsIn the following thread, it has been suggested that the unique Train-ID mentioned above should not be some internal value, but rather a user-defined value.
viewtopic.php?f=6&t=46969 Constant Combinator for Locomotive
Having such a user-defined value would make my suggestion even more useful.