From what I've read, there was no real consistency in what was considered "express locomotive", so Halls could be both lined and unlined - I would guess that there could be a mix of GWR, and G crest W, though I could not say with any certainty. I'd also allow for a mix of Collett and Hawksworth tenders. You're still likely to see wartime black paint jobs. And any locomotive not serviced since before the war would likely carry the Collett Shirtbutton monogram, also as many engines did not see the monogram, it's entirely possible locomotives nearing service time could still be found in Great (crest) Western finish. As with anything, if you can find a dated photograph of something, then it would be allowed - especially if you model that particular locomotive.