Southern Pacific classes MM-2 and AM-2
United States 1911 12 produced
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 with passenger trains, the leading axle was replaced with a bogie in 1914. In the 1920s, they were rebuilt 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.
Technical data
General
| Property | MM-2 | rebuilt AM-2 |
|---|---|---|
| Built | 1911 | 1914 |
| Manufacturers | Baldwin | |
| Wheel arrangement | 2-6-6-2 (Mallet Mogul) | 4-6-6-2 |
| Gauge | 1,435 mm (Standard gauge) | |
Dimensions and weights
| Property | MM-2 | rebuilt AM-2 |
|---|---|---|
| Wheelbase | 15.65 m | 16.71 m |
| Total wheelbase | 25.93 m | 27.05 m |
| Service mass | 174.5 t | 192.4 t |
| Adhesion mass | 145.6 t | 161.9 t |
| Total mass | 258 t | 294.9 t |
| Rigid wheelbase | 3.35 m | |
| Axle load | 24.3 t | 27 t |
| Water capacity | 37,854 l | 45,425 l |
| Fuel capacity | 12,113 l (oil) | 14,385 l (oil) |
Boiler
| Property | MM-2 | rebuilt AM-2 |
|---|---|---|
| Grate area | 6.503 m² | |
| Firebox surface | 21.832 m² | 31.959 m² |
| Tube heating surface | 469.811 m² | 373.749 m² |
| Evaporative heating surface | 491.643 m² | 405.708 m² |
| Total heating surface | 491.6 m² | 500.7 m² |
| Superheater surface | 94.947 m² | |
Power plant
| Property | MM-2 | rebuilt AM-2 |
|---|---|---|
| Driver diameter | 1,600 mm | |
| Boiler pressure | 13.8 bar | 14.5 bar |
| Expansion type | compound | simple expansion |
| Cylinders | four, HP: 635 x 711 mm and LP: 965 x 711 mm | four, 559 x 711 mm |
Power
| Property | MM-2 | rebuilt AM-2 |
|---|---|---|
| Power source | steam | |
| Estimated power | 1,790 kW (2,434 PS) | 2,386 kW (3,244 PS) |
| Starting effort | 293.2 kN | 341.6 kN |
| Starting effort simple | 351.8 kN | |
Calculated values
| Property | MM-2 | rebuilt AM-2 |
|---|---|---|
| Factor of adhesion | 1 : 4.87 | 1 : 4.65 |
| Optimal speed | 37 km/h | 43 km/h |
| Power-to-weight ratio | 10.3 kW/t | 12.4 kW/t |
| Heating Surface Equivalent | 600.8 m² | 755.4 m² |
| Cylinder volume/HSE | 1 : 1,615.90 | 1 : 923.59 |
| Adhesive weight/Total | 1 : 1.20 | 1 : 1.19 |
| Tractive force/Heating surface | 1 : 12.46 | 1 : 14.25 |
| Grate area/Heating surface | 1 : 75.60 | 1 : 76.99 |
| Firebox/Tube surface | 1 : 21.52 | 1 : 11.69 |
| Total/Superheater | 1 : 4.27 |
