Hi,
JJJWegdam noted, that the usage of the tag usage=main/branch differs between Geramany and other countries (see https://www.openstreetmap.org/changeset/20641783).
In Germany, it is usually applied based on the legal status of a line as branch line or main line. This means, that sometimes single track lines with low speed and rather local relevance are tagged usage=main.
What's your opinion?
Regards, Micha