In Simple Terms, Pun Intended, before Superheating applied to Non-Articulated 2 Cylinder Steam Locomotives, Compounding was sometimes used.
Many Different Systems of Compounding were developed depending on Locomotive Builder.
Usually, on a 2 Cylinder Compound Locomotive, one side of the Engine had a Small High Pressure Cylinder which received Steam from the Boiler and Exhausted it’s Steam via an Accumulator in the Smoke Box into a much Larger Low Pressure Cylinder on the Opposite Side.
This System would give only TWO 2 Exhausts Chuffs per Driver revolution.
On Starting, a Compound could be operated Simple with Boiler Pressure to BOTH Sides.
When Superheating developed, a lot of Two Cylinder Compounds received new Simple Cylinders, others worked out their lives and Scrapped.
The Question is; Are there any Two Cylinder ‘Big and Little’ Standard Gauge Tender Engine Compounds Extant.
There is a C. 1900 Narrow Gauge Vauclain Compound Extant in the Yukon.
While not standard gauge and not in the USA, I would be very surprised if a two cylinder compound did not still exist in Argentina. They were very common on the broad gauge (5’6" = 1676mm) in the South and West of that country as compounding was popular at the time of expansion of the railways there. I may check and get back to you!
F. Loree of the D&H was a real proponent of compounding of non articulated locomotives. Under his tenure the D&H had a series of very high pressure 2-8-0s that used the steam 3 times. The last locomotive in the series was a 4-8-0 that used the steam 4 times before exhausting.
There was an article in Trains about these sometime in the 1970s.
D&H 1403, a 4-8-0, was the only triple expansion locomotive in service in the United States. Steam was first in a high-pressure cylinder under the engineer, exhausted to an intermediate pressure cylinder under the fireman, which then exhausted to the low-pressure cylinders at the front. D&H 1400-1402, all 2-8-0’s, were all distinct designs but were cross-compounds.
The article was from a three-part series about D&H steam titled “Consolidations, Inc” which appeared in TRAINS in 1966 or 1967.