2-6-6-2 Steam Locomotives in Mexico

Class HR-01 (Locobase 4430)

Data from narrowmind.railfan.net, which reproduces an older "Fig. 3333", later considerably supplemented by NdeM 1926ca Locomotive Diagrams NG supplied in December 2010 by Allen Stanley. Alco works numbers were 67706-67711 in February 1929, 68644-68645 in July 1934, and 68773-68774 in February 1937.

The 1928 batch had its bell located between the steam dome (which is over the two leading drivers of the rear set) and the forward sand dome.

The four later locomotives had the bell positioned over the smokebox, just in front of the Elesco feedwater heater. Its place on the boiler has been taken by another sand dome. Inevitably, the later engines 5 1/2 short tons more on the drivers, raising adhesion weight to 172,500 lb (78,245 kg). Engine weight grew to 216,000 lb (97,976 kg).

In any event, NdeM got for their money a nice balance of power and speed on a narrow gauge.

Class HR-2 (Locobase 5715)

Data from December 1908 American Engineer & Railroad Engineer (AERJ). See also DeGolyer, Volume 32, pp. 315+. Works number was 30003 in October 1908.

AERJ noted that this locomotive was very similar to the pushers delivered to the Great Northern in 1906. The Tamasopo Division was curvy (15-22 deg), steep (up to 3% grades), and relatively lightly laid (85-lb rail).

Mexican Central followed this singleton with an order for 20 more in 1910-1911, which suggests that this was a successful prototype. See Locobase 11061 for the production engines.

Class HR-3 (Locobase 11061)

Data from "Oil-Burning Locomotives", Record of Recent Construction #57 (Baldwin Locomotive Works, 1911), p.32. See also DeGolyer, Volume 36, pp.. 170; and "Mallet Locomotives with Hobart-Alfree Valves and Cylinders", American Engineer and Railroad Journal, Volume 85, No 6 (June 1911), pp. 239-240. Works numbers were 35754-35757, 35786-35789 in December 1910, 35899-35902, 35997 in January 1911, and 36001-36007 in February.

These were the production variant of the Mexican Central prototype shown in Locobase 5715. At least at the time of their deliveries, these locomotives were fitted with the rare Hobart-Allfree cylinder and drive gear. See Locobase 7820 for a detailed description of these double-walled cylinders, whose valves resemble, as far as Locobase can determine, later poppet-valve designs.

In the same period, the MI was absorbed into the Nacional de Mexico (if the lettering on the tender illustrated in Baldwin's "Record ..." is a good guide), but the engines weren't renumbered until 1930. (By that time, 1115 had been retired, possibly wrecked, in April 1923).

The rest of the class served the NdeM until the late 1950s and early 1960s.

Class HR-4 (Locobase 3167)

Data from NdeM 1961 Locomotive Diagrams supplied in December 2010 by Allen Stanley.Works numbers were 69001-69008 in November 1937.

Giving the impression of "being a slightly under-scale Challenger" [4-6-6-4], these standard-gauge freight engines were modern in every respect. Thermic syphons accounted for 95 sq ft (8.83 sq m) of the direct heating surface. The NdeM's 1950 diagram book showed a slightly lower adhesion weight of 318,000 lb (144,243 kg) and 394,000 lb (178,716 kg) for the engine in working order as a whole.

Very few Prairie Mallets were built as simple-expansion locomotives, Drury (1993) adds, andthe NdeM stud was the largest single class. Alco produced no more 2-6-6-2s after this octet.

They served until 1963.