Dennis O'Berry's MUDHENS book is a good place to begin. A good overview of the history of this class.
The D&RG ordered the Class 125 locomotives in 1903 at a time when compounding was in vogue in North America. Delivered that same year, they were Vauclain compounds, using saturated steam. Saturated steam had been the industry standard from the time of Trevethick and Stephenson, though by 1903 some theorists were considering superheating steam. Theoretically, using the steam twice before exhausting it up the stack provided efficiency by getting more work out of each pound of coal and water. In fact, there were measurable savings, but the economics of railroading were very different in Europe and North America. In really crude, over-simplified terms, fuel was expensive in Europe, while skilled labor was relatively inexpensive; in the USA the opposite situation applied - fuel was relatively inexpensive, and skilled crafts - shop men, engine crews - were much more costly.
The D&RG found that the Class 125 locos did indeed produce more work than the smaller power, and compounding was helpful - but - the railroad tended to overwork the locomotives by putting more train behind them than they could easily handle. Enginemen responded by cheating - using the starting valve to admit high pressure steam into both the high pressure and low pressure cylinders. Acceptable for starting a train, it was hard on the machinery if the locomotive ran for extended distances with all four cylinders using high pressure steam. So trains got over the road, but the Class 125s started spending much more time in the shop.
After two years - in 1905 - the D&RG decided to try converting one of the Class 125s to a simple loco with what were then the standard slide valves. The locomotive was to remain a saturated steam loco, probably for reasons of economy - keep the conversion cost down. All of the narrow gauge power used saturated steam and slide valves, this would be a return to the tried and true technology. The simpled Class 125 was more robust, and spent less time in th3e shop, so the management deiced to do all of them on a gradual basis. O'Berry notes that records indicate it was not a rapid process, with the last Class 125 being simpled sometime between 1912 and 1916.
Another fuel saving technology was under development in 1903. Superheating the steam, so that it was a hotter gas, and would provide more work per pound - and would allow the use of less fuel per mile. Superheat, however, required different valves to work effectively - the slide valves were not as desirable as piston valves. By 1910 most new main line power came with piston valves, and more and more railroads were recognizing that superheat and piston valves attained fuel efficiencies more or less comparable to compounding and saturated steam. In North America, where coal was inexpensive, and skilled labor costly, the compromise of superheat and piston valves, but simple engines proved to be the economically preferred combination. [We won't worry about Mallets - another topic for another day, but note that the L-131s, like the NP Yellowstones and UP Big Boys were all simples]
After World War I the D&RG was in financial trouble, and in 1921 it w2as reorganized as the D&RGW. The new company set about making improvements which it hoped would lead to greater operating efficiency, and a return to profitability.
In 1923-1924 the first four of the soon to be Class K-27 were superheated by the D&RGW These proved to be more economical on fuel. The D&RGW gradually rebuilt another seven with piston valves, all but one with superheat. No. 462, the last piston valve conversion, in 1929, was never superheated. Four remained as saturated steamers with slide valves - 450, 451, 457, and 460. Business was down, the Rio Grande had the K-28 and K-36 class, both less than ten years old, so there was plenty of power for the main lines. Probably the conversion of the last four mudhens seemed pointless, with 20 relatively new superheated mikes on hand. Note that photos indicate that only 460 saw much service after 1929.
Charlie Mutschler
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