You connect the train station with a red or green wire and set it to 'Get train content' instead of 'Send to train'
That would be extremely limiting, because you need both in many realistic scenarios (being able to read from the train and also to write to the train station to trigger a circuit condition).
We need an input and an output port on the train station to fully integrate it into the circuit network. The input allows directing the trains, and the output gives you status information like cargo content, presence of the train at the station, how much fuel it contains etc.
The SmartTrains mod (that allows all this) does have a combinator which acts as the output that you read from, and a lamp which acts as the input that you can use to tell the train where to go (in 0.13 you also have the train station proper as an input for the leave condition).
Being able to connect to the rails in order to read the wagon content -- even from individual wagons -- is not a bad idea either. You could also use that principle for all other sorts of tricks, such as counting the number of wagons, reading fuel content from the locomotive, count the number of locomotives etc; even if there isn't even a station. You could create stops at junctions using rail signals, and then examine how the train looks like that you've just stopped and redirect it appropriately.
I'm all for making the rails connectable and smart. Great idea, now that I think about it (is there a mod that allows it?)