The reference for locomotives and railcars
Southern Pacific classes MM-2 and AM-2
United States | 1911 | 12 produced
No. 4206 already with bogie but before rebuilding to AM-2
No. 4206 already with bogie but before rebuilding to AM-2
collection Taylor Rush

One of the first classes of cab forward locomotives of the Southern Pacific were Mallets with a total of six coupled axles and one leading and one trailing axle each. Their designation was thus MM-2 for “Mallet Mogul”. In order to improve the running characteristics when operating in front of passenger trains, the leading axle was replaced with a bogie in 1914. In the 1920s, they were converted to simple steam expansion, with a larger boiler and superheater. Despite the new wheel arrangement, the designation was now AM-2 for “Articulated Mogul”. They were retired between 1946 and 1948.

VariantMM-2rebuilt AM-2
Axle config2-6-6-2 (Mallet Mogul) 4-6-6-2 (Mallet) 
Gauge4 ft 8 1/2 in (Standard gauge)
Dimensions and Weights
Wheelbase51 ft 4 in54 ft 10 in
Fixed wheelbase11 ft
Total wheelbase85 ft 1 in88 ft 9 in
Service weight384,800 lbs424,200 lbs
Adhesive weight321,000 lbs356,900 lbs
Total weight568,800 lbs650,200 lbs
Axle load53,500 lbs59,500 lbs
Water capacity10,000 us gal12,000 us gal
Fuel capacity3,200 us gal (oil)3,800 us gal (oil)
Power sourcesteam
Estimated power2,800 hp (2,088 kW)3,500 hp (2,610 kW)
Optimal speed27 mph29 mph
Starting effort65,915 lbf76,795 lbf
with start valve79,098 lbf
Power Plant
Driver diameter63 in
Boiler pressure200 psi210 psi
Expansion typecompoundsimple
Cylindersfour, HP: 25 x 28 in
and LP: 38 x 28 in
four, 22 x 28 in
Grate area70 sq ft
Firebox area235 sq ft344 sq ft
Tube heating area5,057 sq ft4,023 sq ft
Evaporative heating area5,292 sq ft4,367 sq ft
Superheater area1,022 sq ft
Total heating area5,292 sq ft5,389 sq ft
Calculated Values
steam locomotive
cab forward
last changed: 04/2023

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