Want to circuit control your stations based on what trains stop there? Train ID's are one solution, but they frequently change when adding or removing rolling stock. Worse, each train is different, which makes them a difficult solution to use when using multiple trains to serve the same routes and stations.
Your can color code your trains based on what kinds of cargo they carry and the stations they serve, but there's no way to use that information to control your stations and trains. Until now.
With Train Color Signal, once you set your circuit-connected station to "Show stopped train" the station will output the color of the first locomotive in your stopped train as R, G, and B signals, in addition to the normal output of the train's ID as the T signal.
Note that the signal is only sent when the train is in Automatic mode, and only changes when a train first arrives at a station. If you change the color of a train that it already at a station, you can set it to Manual and then back to Automatic to update the signal output from the station.