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.

 

Sorry for the late reply to this comment, thinking about it, it also depends on the characteristics of the traction motors. A shunt wound DC motor, fed from a constant DC voltage supply is essentially a constant speed machine, so the motor would only have to be running a few percent faster than the other three for the back emf to build up and reduce the armature current and the resulting torque. A locomotive is more likely to have series wound motors, because they have better starting characteristics, but if the axle slips, the reduction in armature current will also give a reduction in field current, limiting the back emf, so the motor will tend to "run away" more when slipping. It's also possible to have compound wound motors (nothing to do with compound steam engines) which have both shunt and series field windings and a characteristic in between the two extremes.

I'm also thinking of the effect of weight transfer away from the leading axle, and if I remember correctly the 1500V DC EM1s on the Woodhead line had the buffers and couplings mounted on the bogies, with the two bogies connected by a drawbar. I think the effect of this was to reduce the weight transfer between the bogies. I don't know whether the feature has been used on other locomotives.