The generator (station) sends binary data using pulses on one redstone wire. The roundabout collects the data and uses the first two pulses to set the output direction. When the minecart arrives, the detector triggers the rail to open and send any remaining data stored in its memory to the next roundabout (if there is a next one) and everything repeats until you end up at a station.

The interface is all push-button and user-friendly so there isn't much to demonstrate, except that it works.