GWR four-cylinder arrangement?

That's a comprehensive summary of the usage of 2-cylinder compounds in England. But for the full UK picture, don't forget Bowman Malcolm's engines for the Belfast & Northern Counties. The last of the 3-ft gauge compound 2-4-2Ts was in use until 1954.

https://en.wikipedia.org/wiki/Steam...03)_&_Northern_Counties_Committee_(1903–1949)

At the height of its popularity at the end of the 19th Century, the 2-cylinder compound was being constructed in large numbers in many countries, particularly in Continental Europe but even in the USA. Although I don't know whether there were any in Denmark to inspire @Hermod!

The system lost favour after 1900, as increasing loco size and train speeds exacerbated the inherent problems of balancing and of finding space for the large low-pressure cylinder. Some railways moved onto the 4-cylinder compound (the 3-cylinder compound was rare outside UK), but the arrival of the Schmidt fire-tube superheater reduced the popularity of all forms of compounding in most countries outside of France.

There has been as lot of interesting debate on this thread, but not much has been said about 4-cylinder arrangements, on the GWR or anywhere else! Still no sign of the Mid-Hants or the Swanage looking at a new-build Drummond T14!

 

Dont forget the GNR(I) Loco's

https://www.steamtrainsireland.com/rpsi-collection/5/no85-merlin

 

Compound road engines were fairly common, but the simpler single cylinder was more common for agricultural use. Strangely the compound traction engines that looked better on paper tended to be quite shocking machines whilst the not so good on paper were often more pleasant.