The Fernbahnhof is the furthest from the terminal building. and is part of "The Squaire" complex where the Hilton and HGI are located. You don't have to walk outside to get to it. And the easiest way to access it is going up the escalators across from the A and B ticket counters (departures level), which takes you to a pedestrian bridge heading towards the Marriott and Sheraton, and then down a corridor towards the Squaire complex.