Mexican Railway 4-6-2 Locomotives in Mexico


Class Details by Steve Llanso of Sweat House Media

Class M (Locobase 13641)

Data from MexRy 1939 Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. See also DeGolyer, Volume 36, p. 177; and "Pacific Type for the Mexican Railway," Volume 24, No 2 (February 1911), p. 56. Works numbers were 35475-35477 in November 1910.

Drury (1993) comments that this Pacific trio were light and low-drivered machines, "...but low drivers were what the 4.7% grade called for." A desire to control slipping may also explain the low boiler pressure. The locomotives were fitted with a Baldwin moderate-temperature smokebox superheater designed by Samuel Vauclain when delivered.

All three were later equipped with Schmidt firetube superheaters; see Locobase 6743.


Class MR (Locobase 6743)

Data from MexRy 1939 Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. Works numbers were 35475-35477 in November 1910.

Superheating the three small Pacifics that came on the road in 1910 (Locobase 13641) involved a new boiler with 148 fewer tubes in the count and each tube of smaller diameter than the original saturated-boiler setup that included a Baldwin moderate-temperature smokebox superheater. The substantial upgrade included Nicholson thermic syphons

The MR diagram credits the evaporative heating surface area as 2,417 sq ft (224.55 sq m). This is much too low. Taking both the tube and flue surface areas, Locobase calculates a total of 2,675 sq ft. The fireboxes also gained 52 sq ft (4.83 sq m) of Nicholson thermic syphons in addition to the 38 sq ft (3.53 sq m) supplied by arch tubes. An Elesco feed water heater warmed the water before it entered the boiler. Reverse gear was the Alco Type G.

NB: The diagrams do not show superheater area. The figure given in the specs is calculated based on similar installations on other Pacifics of identical flue counts and lengths with known area. The results here agree with the data supplied by Iowa State's Dr Jonathan D H Smith at [], last accessed 6 November 2016.


Class MR-3 (Locobase 6744)

Data from MexRy 1939 Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. (Thanks to Helmut Dahljhaus for his 20 March 2023 email correcting the works numbers.) Works numbers were 67557-67560 in April 1928 and 67666-67667 in October.

It was somewhat unusual for a North American railroad to persevere with the three-cylinder layout more than a few years after its introduction. The Mexican Railway broke that mold in two ways: it ordered three more MR-3s ten years after the first four had come on the road, and those locomotives remained in service as three-cylinder engines into the 1960s.

The 1928 quartet came from Schenectady (

Montreal Locomotive Works supplied the other three in 1938 (Alco works #69061-69063); these weighed about a short ton more. All had their firebox heating surface supplemented by 13 sq ft (1.21 sq m) of arch tubes and 63 sq ft (5.85 sq m) in the 1928 locomotives) or 59 sq ft (1938) of Nicholson thermic syphons. The 1928 pair used Elesco feed water heaters while the Canadians were fitted with Coffins.

NB: The diagrams do not show superheater area. The figure given in the specs is calculated based on similar installations on other Pacifics of identical flue counts and lengths with known area. The results here agree with the data supplied by Iowa State's Dr Jonathan D H Smith at [], last accessed 6 November 2016.

Principal Dimensions by Steve Llanso of Middle Run Media

ClassMMRMR-3
Locobase ID13641 6743 6744
RailroadMexican RailwayMexican RailwayMexican Railway
CountryMexicoMexicoMexico
Whyte4-6-24-6-24-6-2
Number in Class337
Road Numbers120-122120-122130-133, 134-136
GaugeStdStdStd
Number Built337
BuilderBaldwinMexican Railwayseveral
Year19101928
Valve GearBakerBakerBaker
Locomotive Length and Weight
Driver Wheelbase (ft / m)11.83 / 3.6111.83 / 3.6113.83 / 4.22
Engine Wheelbase (ft / m)31.33 / 9.5531.33 / 9.5534.50 / 10.52
Ratio of driving wheelbase to overall engine wheelbase 0.38 0.38 0.40
Overall Wheelbase (engine & tender) (ft / m)60.58 / 18.4663.92 / 19.48
Axle Loading (Maximum Weight per Axle) (lbs / kg)
Weight on Drivers (lbs / kg)141,000 / 63,957138,536 / 62,839147,998 / 67,131
Engine Weight (lbs / kg)216,000 / 97,976219,424 / 99,529249,997 / 113,397
Tender Loaded Weight (lbs / kg)120,000 / 54,431120,000 / 54,431129,800 / 58,876
Total Engine and Tender Weight (lbs / kg)336,000 / 152,407339,424 / 153,960379,797 / 172,273
Tender Water Capacity (gals / ML)6000 / 22.736000 / 22.736000 / 22.73
Tender Fuel Capacity (oil/coal) (gals/tons / Liters/MT)2500 / 94632500 / 94632500 / 9463
Minimum weight of rail (calculated) (lb/yd / kg/m)78 / 3977 / 38.5082 / 41
Geometry Relating to Tractive Effort
Driver Diameter (in / mm)63 / 160063 / 160069 / 1753
Boiler Pressure (psi / kPa)155 / 10.70180 / 12.40200 / 13.80
High Pressure Cylinders (dia x stroke) (in / mm)23" x 28" / 584x71123" x 28" / 584x71120" x 28" / 508x711 (3)
Tractive Effort (lbs / kg)30,976 / 14050.4935,972 / 16316.6441,391 / 18774.66
Factor of Adhesion (Weight on Drivers/Tractive Effort) 4.55 3.85 3.58
Heating Ability
Tubes (number - dia) (in / mm)318 - 2.25" / 57176 - 2" / 51160 - 2" / 51
Flues (number - dia) (in / mm)32 - 5.375" / 13732 - 5.375" / 137
Flue/Tube length (ft / m)19.50 / 5.9419.50 / 5.9420 / 6.10
Firebox Area (sq ft / m2)176 / 16.35272 / 25.27259 / 24.06
Grate Area (sq ft / m2)47.50 / 4.4147.50 / 4.4152 / 4.83
Evaporative Heating Surface (sq ft / m2)3813 / 354.372947 / 273.882823 / 262.36
Superheating Surface (sq ft / m2)388 / 36.06741 / 68.87760 / 70.63
Combined Heating Surface (sq ft / m2)4201 / 390.433688 / 342.753583 / 332.99
Evaporative Heating Surface/Cylinder Volume283.28218.95184.87
Computations Relating to Power Output (More Information)
Robert LeMassena's Power Computation7363855010,400
Same as above plus superheater percentage802510,26012,584
Same as above but substitute firebox area for grate area29,73558,75262,678
Power L1989715,41216,505
Power MT464.24735.79737.59

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