Nacional de Mexico 4-8-0 Locomotives in Mexico

Class N-10 (Locobase 9488)

Data from NdeM 1946 Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange.

Small, low-drivered Twelve-wheeler that served the NdeM for almost 35 years until its retirement in November 1931. They were rated by the NdeM to haul a train of 1,780 tons in 44 cars on the level and 118 tons in 3 cars up a 4% grade.

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Class PR-7 (Locobase 1381)

Data from DeGolyer, Volume 74, pp. 427+. Works number was 57988 in September 1924.

Drury (1993) notes that the KR-1 Mikados, the MR-6 Pacifics, and this PR-7 all had the same boiler (tubes, grate area, firebox identical); only the PR-7 had an Elesco feedwater heater. The 5-A used 14" (356 mm) piston valves to supply steam to the square (bore equalled stroke) cylinders.

It retired in 1962.

Five PR-8s delivered in 1935 were similar, but fitted with a larger boiler; see Locobase 11952.

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Class PR-8 (Locobase 11952)

Data from NdeM 1950 Locomotive Diagrams supplied in December 2010 by Allen Stanley; and D[ennis] Rock Carling, 4-8-0 Tender Locomotives (New York: Drake Publishers, Inc, 1972), p.29 and Table IV, #54. See also George H Drury, Guide to North American Steam Locomotives, Revised Edition (Waukesha, WI: Kalmbach Books, 2015), p. 233. Works numbers were 61867-61871 in October 1935.

Locobase 1381 described the lone PR-7 that Baldwin supplied using the same boiler design as the 2-8-2 and 4-6-2 locomotives delivered in the same year.

While it was not unheard of in Europe and South America to use the Twelve-wheeler arrangement for passenger service, it simply wasn't done in North America.

Yet, the Baldwin quintet offered reasonable speed and more adhesion weight than a similar-sized Pacific or Hudson. It had an unusually square bore, which several other eight-coupled designs of similar size and power also deployed. Its generous firebox heating surface area included 69 sq ft (6.41 sq m) of thermic syphons. Photographic evidence shows that the class hauled both freight and passenger trains.

A key feature was the use of drivers of 10" (254 mm) greater diameter than any other North American Twelve-wheeler. Very few other 4-8-0s around the world rolled on such tall drivers.

George Drury speculated about the NdeM's motivation and Baldwin's response:

"Perhaps ...the railroad thought about replacing the trailing truck of the Pacific with a fourth pair of drivers to use the weight of the firebox of the Pacific for tractive effort ...To provide room for the drivers under the firebox, Baldwin raised the boiler 6" [152 mm] taller than the Pacifics and 9" [229 mm] taller than the Mikados."

D Rock Carling commented that the PR-8s were "larger and more powerful than any of the US 4-8-0s." He also noted "one unusual consequence possible only because the locomotives were oil burners: the tops of the trailing coupled wheels were just inside the wide firebox!".

Drury added "The resulting locomotive apparently was successful, and in 1935 Baldwin delivered five more with the same running gear and a slightly larger boiler."

3001 and 3003 were retired in 1962; the other three followed in September 1963.