The Class 320 is used on routes out of Glasgow Queen Street station, and also on Balloch to Airdrie routes and on some other Strathclyde routes. The nearest for TS2019 would be sections of the Glasgow to Edinburgh route from Steam, or the Balloch to Airdrie routes from third parties. Armstrong Powerhouse provide Class 320 /321s with the correct liveries for these routes. Note that the Steam version of the Edinburgh to Glasgow route is not fully electrified.
A fully electrified route for the Class 320 would be the West Coast Main Line North running from Glasgow Central station to Carlisle. The 320s operate on that line between Glasgow and Motherwell, Airdrie and elsewhere on or near that route. No problems there with lack of electrified routes.
Something like this? http://www.suburban-glasgow.org.uk/ https://steamcommunity.com/sharedfiles/filedetails/?id=907769903 Read the notes and what the requirements are FIRST