In this case, yes. But then you have to change this for many lines in Germany, if you take a look on OSM-based www.openrailwaymap.org ;-)

Because it's operated as a main line. That has nothing to do with it's importance for over regional transport. See also Wikipedia saying it's a main line: https://de.wikipedia.org/wiki/Bahnstrecke_T%C3%BCbingen%E2%80%93Sigmaringen