With the length of the summertime WHL trains, plus the incoming loco with the stock, even a single loco only just seemed to fit into the Queen Street layout anyway. Although I did hear of the trains being used as a running-in turn by St Rollox, with a separate crew and likely an engineer or two on board as well. Bad luck to the train engine if a defect occurred though. It seems very different to how Swindon did running in, down the Badminton line, reverse round the Stoke Gifford triangles, and if all looked OK a full blast return back home for lunch.
I actually haven't been down the line since Class 27 days, but the summertime loads then, American Interrail-ing student tourists, troops of scouts with full camping kit taking up much of the van (and there was generally more than one van), Glaswegian buffet steward's choice words as he was overwhelmed by the queue, etc, all just seems improbable to fit into the multiple units of nowadays.