We don't see much Concertina stock on GWR layouts, perhaps because they were 70 foot corridor mainline stock and thus quite large vehicles. When introduced in 1906, their range included the Paddington-Neyland expresses (where some of the Brake Thirds ran right up to 1956!) and other premier services. However, some of the Tricompo slips were in fact used as individual coaches on branch services from 1941 (with slip gear removed). Here is a selection from the small range of 4mm kits available from Marc Models. You can see more on their own website.