Cybernetic combinators should provide a color signal to reflect their current state.

My proposed set of signals:

No requests/station empty: cyan
Request exists: yellow
Train is on the way: blue
Train is at the station: green
Pathing error (train never reached destination, train cannot find a path, no train exists): red
Cargo error (train left requester with cargo, train entered depot with cargo, train is being held due to cargo): magenta

Having this output would be very useful, whether it's to simply run a lamp next to each station so you can see what's going on from remote view, or to more precisely control production.