Why would the railways to Sassnitz and Sassnitz Fährhaven be main lines? They're single track lines with a speed limit lower than 100 km/h. What is your source?

Warum sind die linien nach Sassnitz und Sassnitz Fährhaven usage=main linien?

This is common knowledge for most lines, see Wikipedia for instance: https://de.wikipedia.org/wiki/Bahnstrecke_Stralsund%E2%80%93Sassnitz

In General: main/branch line are defined within the German railway regulations.

German branch lines aren't defined by their importance, but by the possibility to vastly simplify their equipment:
- limited or no signalling
- simplified operational procedures
- lower line categories
- level crossings without active protection
- maximimum speed of 60 km/h (sometimes 80 km/h and rarely 100 km/h)
- steeper gradients, lower curve radius
- ...
Most German branch lines were abandoned or upgraded to main lines, nowadays the majority of railway lines are main lines.

This German definition differs from the international definition. usage=* When looking on OpenRailwayMap, Germany is clearly deviating. Wouldn't you agree it would make more sense OSM used the international definition, rather than a different definition in each country?

This is a general issue. I transfered the discussion to the OpenRailwayMap mailing list.

Valid point. Thanks

I think - main and branch - it should be defined by the country government (the Ministry of Transport or Transport Regulator Agency). But for example in Slovakia, it is not a case. We have officially five categories, the first and second are "main" and the third, the fourth and the fifth are "branch lines". But I know several cases of the second class railway lines (single-track lines) that have usage=branch.