"these routes use the same trunk line through the city centre, so the North Clyde line or Argyle line."

That's two trunk lines. If we're grouping by trunk line (which I don't agree with, but will adjust to if that's the concensus), the Argyle Line (services going via GLC Low Level) need to be under a separate ref/route_master.

However, such groupings are not really appropriate. OSM seeks to represent data as it appears on the ground. There are no references to the Argyle Line or North Clyde Line in any ScotRail's public documentation nor as signage at stations (only terminuses/terminii are publicly accessible). Argyle Line and North Electric Mainline are used by Network Rail - not ScotRail or National Rail.

Regarding routes for separate platfoms. The alternative (without creating broken relations) is that OSM wrongfully implies that a service always leaves from a particular platform. Or that the route ways within the relation need to cease at the last switch before the station in question (in the case of services departing/arriving the Western end of Edinburgh Wavery, this would the the Princes St Gardens junction. For Glasgow Central High Level, this point would be virtually impossible to determine.).

osm.wiki/Public_transport#Service_routes: "The list of stops and platforms is followed by an ordered list of the ways which are used by the vehicle from the starting station to the terminal station. Each direction of the route must have its own relation; no forks are permitted. If there are multiple different routes from the starting station to the terminal station, each variant must have its own relation." Adding all platforms that a service could stop at would necesarilly require a fork to be created, which is disallowed under PTv2. Simply adding the plaform ways and/or stop_position nodes would similarly violate this constraint (as they would not be accessible from the route way).

Also, is "North Clyde Line" really an apporiate ref or top-level route_master relation for service routes? Would seem to me to be more applicable to the physical railway infrastructure.

PTv2 would suggest that route_masters should be used to group routes that start and stop at the same terminus, or maybe routes that stop part-way towards one or other terminus.

Cheers, hadn't realised that the key was case sensitive. Should be fixed now.