Miss Prism

When you say 'shake', do you mean wobble from side to side?

SRM 63 in what is thought to be brown livery.  (There was an RMweb discussion about this somewhere, iirc.) It retains its below-waist panelling. The Kernow has below-waist panelling, because that is what the preserved 93 has. It's engine compartment windows have been updated. (Again, like the preserved 93, although the prototype 93 never had the early-style windows.) It has an original low bunker.

Here is 63, later, in crimson lake. Date unknown. All its below-waist panelling has gone. Bunker state unknown. Front sandboxes are not yet in front of the bogie frame. My guess is c 1920.

I can't find any evidence of whistle shields.

I think that's Castle 100 A1 Lloyds on a running-in turn. (BTM to Swindon?)

1 hour ago, stevel said:

And no front step or higher handrail on the 1873. It is a minefield of differences.

Yes.

When the filler moved forward on later (3-segment or 5.5 segment) tanks, front steps and a higher handrail were fitted. Here's 1874, still with old-style lamp spigots, but post-1904 when it got this S4 boiler. (It was panniered in 1912.) Looks like Weymouth.

The most contentious area of saddle tank design is the tank cross-section. Judging by the build span (1890-5), I am reasonably sure the 1854 saddle cabs were 8' wide, but I've have not seen a good end elevation for the class, and the saddle tank end elevations that do exist do not match most of the 1854/2721 photographic evidence. These two pics show how much narrower the 1854 tanks were than the cabs. I would estimate these tanks are 7'2" wide. That said, it's probably certain that some 1854 locos ran with different width tanks at various times. 

There was considerable variation in cab fronts on saddle tanks, even within the same class. Note the extra large spectacles on 1736 (at Laira, 1904). 1873 has a short tank, an old-style cylinder cover and an early injector.

Thanks for pointing to the 1752 pic - I had overlooked that one!

41 minutes ago, sjgardiner said:

What colours of uniforms would GWR crews have worn in the 1912-22 era for this paint scheme?

http://www.gwr.org.uk/liveriesuniforms.html

There should be a handrail on the footplate between (side elevation) the front and middle splashers.

1 hour ago, richbrummitt said:

Good spot. I’d forgotten from when researching my model (still unfinished from a few years ago) that front steps were a ‘modern’ figment. 

I think the front tank step came in with the '5 and a half' segment (extended) tanks, and these date back quite a way. (Not sure exactly when.)

12 minutes ago, Tallpaul69 said:

Can any one explain the use in BR(W) days the use of the black(?) O18s?

I don't think Rapido is suggesting the O18s appeared in black. (If I've understood your point correctly.)

17 minutes ago, Compound2632 said:

(Can one strip the DCIII brake gear off the O18 and turn it into a conventionally-braked wagon, or are there too many differences compared to later diagrams?)

The O24 was the first RCH brake design. On the O24 body however, the corner plates extend over the curb rail. (Unlike the O18.)

I can't find any evidence of a front tank step on a 5-segment 1854.

The lid on the top of the sandbox should be an ellipse, not a rectangle.

Injector  (albeit a modern one)

The rectangular base of the tank lifting rings should be a lot smaller. See http://www.gwr.org.uk/pannier-pics/pannier-topfeed2.jpg

Edit: as you were, some of the early ones were as you have drawn.

The cylinder section of the filler should follow the radius of the tank. This convention may however be a modern (say post-1890) feature.

The dome needs a bit of tweaking I feel. The radius to the tanktop needs to change around the base perimeter. This radius in end elevation should be larger than the radius in side elevation.

The O18 is a good choice, straddling two eras. It was the final DCIII open, and the first to have a sack-truck door.

56xx and 57xx wheels were different. The front of the boss of a 56xx sticks out further than that of the 57xx, i.e. the wheel is thicker at the boss. Not sure why this was - strength presumably.

Visually, it is not an obvious difference. 

2 minutes ago, kevinlms said:

I did read somewhere, that the GWR did need to build new Broad Gauge locos, up to about 5 years before elimination, presumably because of increased traffic requirements and old age of the fleet.

New BG locos were still being built (albeit in 'convertible' form) as late as 1891.

Overhauls at OOC?

Well, yer never know. Absence of evidence does not equal evidence of absence, and I agree with you it would be surprising if Caerphilly did not have some 8750 visitors in the period in question.

On 18/01/2024 at 20:27, Miss Prism said:

I don't know of any evidence of a 57xx/8750 carrying Grotesque font insignia.

Johnster, my apologies, I don't know how I could have missed this. (A moment of madness.)