I think we're going to be waiting a while for info. I notice they've not specified any trains, so maybe they're trying to keep it a secret to surprise us... Or they've just not decided yet. I do hope it includes the branches to Neilston and/or Newton, as well as the Paisley Canal line (or at least part of it) for the depot at Corkerhill or Shields - the cathcart circle itself is only about 9 miles.
The Cathcart Circle isn't flagged as a Rivet development (I expect they're busy with Arosa), but IIRC they produced a Cathcart Circle route for TS.