The 00 Works J15s (both types) would work well for these, but other than very very small number of SSM or JM Design brass kits, there’s nothing GSWR. Mind you, many GSWR branches were operated almost solely by J15s for decades - Mitchelstown, Newmarket, Kenmare and Castleisland come to mind. So a very reasonable layout could operate with nothing but these, just as a 1970 one could with nothing but 141s.
I’ll find a pic to show the livery details. Basically it’s the same as BCDR / LMSNCC / (British) LMS maroon, as opposed to the very much darker “Guinness”-like hue of the GSWR.
Like in earlier CIE times, a train of passenger stock would rarely have more than two vehicles the same. A brand new GSR Bredin could happily trundle along with a six wheeler built in the 1880s, and a 1915-era wooden bogie.
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