- Class 380: North Berwick to Ayr (3h??*) with Scotrail (joined-up services)
Those Edinburgh/North Berwick - Ayr trains haven't run for a while now, not since at least the start of the first lockdown I think. When they did, they ran via Carstairs, a reversal at Glasgow Central, Paisley & Kilwinning. The longest journeys time-wise the 380s do now are Glasgow Central - Ayr stopping services (41 miles & taking 1hr give or take a few minutes) & Glasgow Central - Largs (42 miles and taking 56 minutes on average).
For the ScotRail EMUs that have moved on/been withdrawn:
Classes 303/311: Dalmuir - Lanark/Carstairs (via Yoker, Partick, Central Low Level, the Hamilton Circle & Holytown) & Helensburgh Central - Drumgelloch (roughly 35 miles) are the two that most readily spring to mind, though am sure there were journeys that the 303s transferred to the Manchester area undertook that equalled or surpassed these in duration/distance. Back in Strathclyde, 303s could crop up on golf specials on Glasgow Central - Ayr runs and there was a regularly scheduled evening-peak working for the class from Glasgow Central to Ardrossan Town (31 miles) & back working in the late 90s/early 2000s.
Class 305: Edinburgh - North Berwick, not aware of the class making regularly scheduled appearences at Glasgow Central on the services via Carstairs unless anyone knows any different?
Class 314: Like above, Dalmuir - Lanark/Carstairs (via the Hamilton Circle & Holytown) and Helensburgh Central - Drumgelloch (roughly 35 miles). By the end of their careers, Glasgow Central - Wemyss Bay was the longest run they took time/distance-wise (30 miles and taking roughly 50 minutes).
Class 322: Glasgow Central - North Berwick via Carstairs (78 miles). One 322 (322484) cropped up on a evening-peak Glasgow Central - Ayr working and return in late April 2010, but this was very much a one-off occurence and significantly shorter than Glasgow - North Berwick, so it probably shouldn't be counted here.