Until such time if ever other gcc dlc trains are released are there any current br class trains that you could senario planner onto gcc that wouldn't be totally unrealistic to have ever been seen on gcc?(I'm guessing what ever the max size platform at glasgow station is would limit some train options).
Glasgow has fairly long platforms since terminating WCML trains like the class 390 stop there irl. In terms of trains already in the game that could fit, you have the class 43 HST that would be semi-realistic. You also have the class 47. Keep in mind I'm disregarding proper liveries and era for this explanation. The main problem with your question about GCC is that most of it is not a mainline. It's the Cathcart Circle. So mainline trains that do the WCML services don't run on most of it. We don't have any class 318s, 380s, or 385s in the game yet to run alongside the 314s on the primary part of the represented route.
The HST really isn’t that long in comparison to other trains. I mean on L2B there will be 8 and 12 coach class 377s. Plus, Glasgow Central has serviced HSTs in the past as well as the even longer class 390. The short lived infamous class 370 APT that was 14 coaches long even made a trip Glasgow in the 1980s.
Stream just started looks like both railtour options will be part of the route.+and at least one br66 sevice as well.
Railtour trains normally have 2 locos and 8 intercity coaches and the trains are 199m long so if there are 3 station that this train can start or stop at then the use of longer trains is possible.But i wouldn't be surprised if the end of line platforms where as long as 250m each.
If you use the livery designer you could paint it into BR blue and create a 1980’s scenario. Throw in some 47’s at Central with some 101 dmu’s and freight shuttling around. It will require some imagination with the scenery.