Yes, I had an idea, but that seems to be no longer valid, have to investigate further, seem sto be quite complicated.
I’m also thinking about using ‘operator’ and ‘to’, ‘from’ only after having checked CSV:route_id against OSM:gtfs:route_id to distinguish ???_700 and ???_3 - this way allowing two CSV entries but then also having two route_masters.
Is it worth investigating further assuming ???_3 and ???_700 are bugs in GTFS?