Lots of good history
here of the Midland & Great Northern Joint Railway, which was owned by (you guessed it) the Midland Railway and the Great Northern Railway.
The "Railway Race to the North", a fabled series of events in the 1885-1895 period, was a province of the express Singles. Here's one about to depart Kings Cross on the first leg of the race to Aberdeen.
The Midland coaches, which were apparently often mixed with the teak coaches of the GNR, would have been maroon (= crimson lake) as shown in the photo in my post above.
Most Aster Stirling Singles pull all-teak consists. Like this one on a euro eBay site:
Which reminds me - those curled edge roofs are going to be an issue on a modified "Annie" or IP Engineering coach! Maybe some 1/4 round and tissue paper covering to hide the joints?
Murray Wilson's son Nick was running a similar train at Cabin Fever early this year - it shows up on the videos.
Finally, I note that Hornby made a set of Midland clerestory coaches in 4mm scale. They are a useful reference for color and lining.