Does anyone know how ticketing works for the outer parts of the Köln S bahn? I need to go from Köln HBFto Troisdorf, back to the HBF and out to Troisdorf again. I know Troisdorf is in zone 2b and i've found the single fare but I can't find any mention of return/all day tickets. Do they exist or will I have to buy 3 singles?