My own first time posting on here and you've given me the opportunity to post a wall of text, brilliant. Bit late answering but in case you're still interested in answers for this, I'll target the mid to late 80s as that's where I believe the route is set.
The 80s was the thick of the BR blue & grey era. Almost everything in this time period was in the corporate colours of the prior decade, with the earliest signs of sectorisation starting to appear, some of the first locos and coaches were beginning to be repainted into the Executive red stripe liveries and some of the multiple units were being repainted into regional liveries, think the light blue Pacers and the bright orange GMPTE trains.
MK1s and MK2s in B&G would be absolutely everywhere. Many of them would probably be MK2s, and if not with a BSO then would probably have a MK1 BG on the end in its place. Other MK1s were generally used either to increase formation lengths in peak times, or for extra services such as summer specials or a loco hauled standing in for a disabled DMU.
Locos in an area like Duchy Days would absolutely have seen primarily Class 47s on long distance express, considering that I believe the route is supposed to be set somewhere southwest of London. Like the coaches these would have been something like 50% blue, 40% blue large logo, with a handful occasionally appearing in Executive as it was just being rolled out. It is also possible that occasional Class 43 HSTs could appear among them. Semi-fast services would likely have been pulled by the likes of the 37 and 31, although possibly also increased length DMU formations such as a double set of 117s. All stops regionals by this time would have been a mix of first and second gen DMUs, on this route I generally put 117s, 141s, 142s and 150s on those, as I said earlier with the occasional 37 or something similar pulling a small rake of MK1s standing in for a shortage of units. Though depending on exactly how far into the 80s you intend to aim for, towards the end the backbone of the fleet was almost entirely 2nd gen with the 1st gens covering for their gaps, so loco hauled stand-ins became rarer as time went on. If you wanted to target the very end of the 80s, some of the 2nd gen DMUs could have started to be repainted into Regional Railways liveries, and Class 155s or 156s could perhaps appear for the main line stoppers. For the passenger trains on the Tregullow branch I always have either a 121 Bubble Car or a 141 Pacer.
As an extra note, if you wanted to you could also put in the occasional Class 40. The last of these were withdrawn in 1986 I believe, but taking a little creative liberty I don't find it impossible to see one brought out of storage for something like a seaside special. The 45s were around in the era but all further north so not fitting of this region.
For freight, by this time pretty much the only place where you would see old vacuum braked stock was in departmental use. In active service, this route is set up for primarily clay mining traffic I believe, so a lot of either two or four axle hoppers would be around. On mine I have freight trains spawn pulling Railfreight red HEA hoppers simulating coal trains, but pretty much anything from the Hxx range will fit. For the limestone mines, the PGA two axle will be your best bet. You may have also seen intermodals with shipping containers travelling from one end to the other, I don't believe there is a container port on the map itself so these will probably just be passing on the mainline. The same could be said for tankers, generally I would imagine either larger ones such as TEAs or smaller ones such as TTAs, the latter of which had uses in pretty much any fluid transport and a few different variants for different purposes. PCA concrete tankers are also a possibility, these could be going to sidings around the map as well as simply end to end. A small handful of old standard van type wagons might have been around, or perhaps the later types like the VGA, but with containerisation well underway these would have been few in numbers.
In terms of freight locos, the first thing is that every yard at this phase had a Class 08 shunter, as did every station as its pilot. They would have all still been in blue, possibly one or two might have been repainted into something else such as Railfreight but my gut says that came later, and it was a rare thing anyway as many 08s went all the way to privatisation still in blue. As for the freight locos, all three of the locos I previously mentioned in passenger would also apply here, either in blue or some starting to be repainted into Railfreight grey. Two toned sectorised Railfreight wasn't until a little after that towards the very end of the 80s, on my own rendition of the map I only have it appear on one loco set which is a pair of 37/5s that I have permanently based at one of the open cast mines, and very occasionally on freight passing through from end to end such as on coal trains. Beyond the 31s, 37s and 47s, the 56 could have been sparingly seen, though I personally don't use it. The 58 could appear pretty similarly, and that one I do use on the main line freight. I also have one Class 60 based at the yard in Exbridge.
