Brake Third, Third, Composite, Brake.
Above: Small layouts require short trains. Recently I've been looking at prototype examples of short GWR formations in pre-grouping days, and options for employing them on Farthing. Here are some of the more obvious/common ones to start off with. Above are the classic Ratio 4-wheelers, with an RTR-bashed PBV at the end. The Ratio kits constitute a T47 Bke Third, an S9 All Third, and a U4 Composite respectively. I am not sure why these particular coaches were chosen for the kits, but if joined to a V5 PBV as seen at the back of this train, they form one of the sets built for the Ruabon & Dolgellau line in 1900 (although it is unclear to me whether these sets ever ran individually, or only in multiples?). Add another Composite, and you would have a formation similar to the Cardigan branch train around 1911 - albeit with different diagram numbers. The V5 was built from the ends of two Triang coaches, as described here.
Brake Third, Composite, Brake Third.
Above: The Brake Third / Compo / Brake Third formation was very common on GWR pre-grouping branchlines. There's a prototype example here. In this case the leading Brake Third is a Holden "Metro" coach, of which some were devolved to branch services and mixed with non-Holden 4-wheelers (eg the Faringdon branch set). The "Metro" is a modified Ratio kit using Shirescenes sides - a quick way to add a bit of variety, although it involves a number of compromises (details here).
Brake Third, Composite, Third, Brake Third, Siphon.
Above: This is about the maximum length of train I can reasonably fit in the bay platform at Farthing while still preserving full operational scope. The Brake Third / Compo / Third / Brake Third arrangement was another fairly widespread 4-wheeler formation. It was apparently known as an "A" set in Edwardian times and a "WW" set in the 1920s. In this case I have added a 6-wheeled low-roof Siphon at the end, built from an old K's plastic kit.
Of course, it wasn't all so streamlined! Far from it, in fact, as discussed in this entry. Personally I actually prefer the ungodly mix of different coach styles seen on many GWR trains, not to mention the really short trains that ran on some branches. But more on that later.
Sources: See GWR Branchline Modelling vol 2 by Stephen Williams for a discussion and list of formations on selected branches.
Note: The GWR would have called a Brake Third a "Van Third". I use the former term here as it seems more intuitive.