Originally Posted by
SunshineStay
For Lufthansa they definitely check your ticket on a train route from Cologne to Frankfurt, so skipping that first leg voids your complete ticket.
Nope, the system is not linked for the Airrail system, so you can still show up in FRA and start your journey. Just make sure you checkin online for the QDU/ZWS/QKL/etc-FRA segement.
What will void your ticket is if you don't show up for the SXB-FRA Airport bus. There the bus driver checks you off a list and hands it to someone in FRA. This is because people abused low fares ex France.