I believe that they went no further west than Hyndland back then though, not through to say Helensburgh or Balloch etc
You are correct, trains through Queen Street Low Level in those days were run in (broadly, because the services were irregular) two overlapping parts, Bathgate/Airdrie etc through to Hyndland (not the present station but a terminal stub), and services to Helensburgh/Balloch etc starting from Bridgeton (again not the present station on a different line but another terminal stub). Locos were from different sheds each side, the Bathgate trains were run by Parkhead and Kipps (Coatbridge) sheds, and by Bathgate and Haymarket, to the end of steam often old North British 4-4-0s.
Were there not "Starlight Express" services that ran from Glasgow suburban stations via what's now A2B then either the Waverley route or the ECML to destinations south of the border overnight?
Actually "Starlight Specials". Starlight Express was the 1990s musical, lyrics by Richard Stilgoe, but who had been a youthful railway enthusiast and recalled reading about them in magazines of the era. They did operate from secondary termini, but in Glasgow used St Enoch, and ran by the G&SW to Carlisle, WCML to Bletchley, thence getting to Marylebone in London. They were probably the last Anglo-Scottish services to be fully double-headed steam hauled, and thus got a lot of attention in the enthusiast magazines of the time. Only ran at summer weekends, notably cheap fares. Finished in I think 1962.