My experience in what might be called 'nearly full' steam days was limited to Goring (ie between Reading and Didcot) in late 61 (just a few hours) 1962 66, by which time, of course, it was fully diesel. Plus a few visits to Reading/Didcot.
Referring to early 1962, there were only Warships (D8xx type) on the main line. D6xx types were already Plymouth area only, I think. Maybe D1000 came out in 1961, but I didn't see it in sand colour but I do remember seeing D1002 off works in green. I guess that would have been May or so, 62. It was running light.
My recollection (sorry, lost all the spotting books) was:
I saw not one King working (sob). I think the Kings were largely restricted to PDN Wolves and West of England trains. Whatever, I never even saw one, even when I went to Reading.
I think most of the scheduled expresses were Castles or D8xx. I can't be sure, but sometimes Halls (or Modified Halls) were used on summer extras.
Counties were quite scarce (two were at Didcot, but I think they worked the early morning commuter trains, and back after I'd gone home for tea.!)
Granges too, were not common at all, and Manors at that time well, I never saw one. My impression was that these were kept for the restricted routes in Wales - Cambrian and maybe some in the south.
The relief lines were pretty heavily used, either with local DMU workings or goods. All local workings were DMU by 1962 except, for some strange reason, a Sunday up train (it's a long time ago, I think it was about 10.30 through Goring) which was a Banbury 41xx tank. This returned from Reading at around 12.25 (ex Reading) IIRC. I caught it once and we 'raced' a D8xx Warship starting away from Reading on the fast.
Good traffic was especially heavy in the evening, around 20.00. I think most goods was 28xx, maybe 43xx Moguls and Halls. I don't think we got any of the tanks, neither the big ones (eg 72xx) or the o-6-2Ts (56xx, 66xx) except sometimes from the Banbury direction, perhaps.
I did see some of the 47xx though, I assume these were on fast freights towards Bristol in the evenings, but can't swear to it. But they were not common.
By late 1963-64, things were changing. Diesels displaced Castles, of course, but funnily enough, I think Counties became a bit more common (even as steam was being replaced), I certainly remember seeing 1000, 1010 and 1011. I can't remember Granges becoming more common, but Manors certainly began to appear now and then some were shedded at Reading, and one (was it Frilford?) had GWR on the tender. It was sometimes station pilot at Reading, facing Paddington.
I assume the Manors arrival was because, eg the Cambrian was LMSed with Standard 4s, and the WR did not want to just scrap the 78xx en mass.
Sad to say, I can't really recollect how steam was replaced on freight. I do remember that D63xx were rare, very rare, at Goring in the early days, but then became quite common, probably around 1966.
The memory is muddied, because by around 65 I think GWR steam on the WR workings was dead but steam seemed alive and well, at least in summer, because of the increased number of inter-regional trains with Bulleid pacific haulage from Bournemouth/Poole. Some Saturdays there seemed to be 4 or 5 such workings, which livened up the day no end.
When I first went to Reading, I think it was a Sunday around Easter 62, the shed was stuffed from memory there were about 70 locos there, almost all steam, from Castles and Halls via 43xx and 28xx to pannier tanks. There may even have been the odd 14xx, not sure now. (That may have been Banbury.)
I remember going again in to Reading shed, I think, early January 1965, and there were just 3 or 4 steam in store. The Berkshire paper (Reading Mercury?) that week had a photo of these being towed away from the shed I think it was around noon on the very Sunday I had been there.