I don't know marble but the gpx file "berlin_hbf_txl_marble_export.gpx" looks ok also in gpxsee it loads ok. Maybe something wrong with the marble parsing.

Try to import it without the route part (list of waypoints) and see if you can see the track in marble.

So delete:
and everything in between.

When this is also not working then try to delete following two lines:
and try to fix the indent. So that it is simple and short. But other programs really need the track segment opening and closing.

For the other file berlin_hbf_txl_gpsbabel.gpx the first tracksegment has no elevation value and has a short closing of the trackpoint which could maybe lead to a parsing error in Marble.

Also in gpxsee it looks funny and different to compare to the other gpx file, because there are so many tracks. Looks like that different segments are now tracks.