I’ve noticed that stumbling around for some questions about service=siding for railways that one user has changed some sidings into main tracks i.e. usage=main/branch under the justification that these are important for passengers (Changeset: 132981782 | OpenStreetMap, Changeset: 131667310 | OpenStreetMap).
The justification @DaveF gave is that these are main tracks because they’re intensively in regular passenger service which is different from how I know it in that passenger use doesn’t matter, only that trains explicitly need to request to enter these trains alongside the fact that trains have to slow down to enter these (and for further notes, Dave also put in a complain in the Wiki about the definition as on the Wiki).
The one true edge case where I’m in favour of treating both tracks as the main tracks is when both tracks are unidirectional (e.g. signals are placed in only one direction) after splitting from a bidirectional track (so there is main and a minor track but rather two main tracks) but this also is something I’m unsure of (example).
Relatedly, I also remember a changeset but I’ve checked many other places where “German rail enthusiasts”[1] supposedly mapped and the practices are the same as in Germany, apparently.