How do you use route_relations?

First of all, you are perfectly allowed to just ignore the route_master relation. No software currently uses the route_master relations. They stem from the mindset of collection relations which are deprecated in other tagging domains of OpenStreetMap.

If you insist on making a route master, then do it as follows: Create a new relation. Add all routes that have the same value for the "ref" tag (e.g. "410") and for the "network" and/or "operator" tags as relation members in arbitrary order. Then tag the relation with the tags that have all collected members in common, i.e. at least "ref", "operator", and/or "network".

Public transport usually works with lines. The route master represents such a line. The route relations are all the different variations. It's nice to be able to tie them together. Line identifiers are reused all the time. Without route_master relations, how would one know which line 1 route variations belong together and which are in a different city? In Aalst there are 2x a line 91 and 92 for some odd reason. So it can happen within the same city as well.

As for the question. Just create a route master and add all the route relations for the same line to it.

Jo