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Chicago, Rock Island & Pacific (Rock Island) class M-50
United States | 1913
82 produced
M-50a No. 4058 in June 1952 in Blue Island, Illinois
M-50a No. 4058 in June 1952 in Blue Island, Illinois
Arthur B. Johnson / collection Taylor Rush

One of the first railroads in the USA to purchase the 4-8-2 “Mountain” wheel arrangement was the Rock Island. In 1913, however, only the two prototypes of the M-50 class were initially delivered. They had square cylinder dimensions measuring 28 by 28 inches and drivers measuring 69 inches or 1,753 mm in diameter.

The First World War had to pass before series production came about. Between 1920 and 1924, 80 more M-50 were built in several batches, some of which were fired with coal and some with oil. They also had larger drivers of 74 inches or 1,880 mm, a larger firebox thanks to thermic syphons and a boiler pressure of 200 instead of 180 psi. Some of the coal-fired machines later received a stoker.

In 1941 seven were sold to the St. Louis Southwestern and rebuilt there. The Rock Island also rebuilt 18 of its locomotives to the M-50a. They got roller bearings on all axles, lighter connecting and coupling rods, disc drivers, a boiler pressure of 220 psi and two inch smaller cylinders that were cast in one piece along with the front frame.

Variantprototypesproductionrebuilt M-50a
General
Built19131920-19241941-1944
ManufacturerALCOSilvis
Axle config4-8-2 (Mountain) 
Gauge4 ft 8 1/2 in (Standard gauge)
Dimensions and Weights
Wheelbase38 ft 11 in42 ft 3 in42 ft 1 in
Rigid wheelbase18 ft19 ft 10 in
Total wheelbase70 ft 2 1/2 in88 ft 4 in87 ft 10 in
Service weight333,000 lbs378,000 lbs378,500 lbs
Adhesive weight224,000 lbs255,250 lbs253,500 lbs
Total weight493,500 lbs572,000 lbs722,000 lbs
Axle load56,000 lbs63,813 lbs63,375 lbs
Water capacity8,500 us gal10,000 us gal20,000 us gal
Fuel capacity28,000 lbs (coal)32,000 lbs (coal)5,000 us gal (oil)
Boiler
Grate area62.7 sq ft63 sq ft
Firebox area312 sq ft397 sq ft368 sq ft
Tube heating area3,805 sq ft4,288 sq ft4,069 sq ft
Evaporative heating area4,117 sq ft4,685 sq ft4,437 sq ft
Superheater area944 sq ft1,196 sq ft1,276 sq ft
Total heating area5,061 sq ft5,881 sq ft5,713 sq ft
Variantprototypesproductionrebuilt M-50a
Power Plant
Driver diameter69 in74 in
Boiler pressure180 psi200 psi220 psi
Expansion typesimple
Cylinderstwo, 28 x 28 intwo, 26 x 28 in
Power
Power sourcesteam
Estimated power2,800 hp (2,088 kW)3,300 hp (2,461 kW)3,500 hp (2,610 kW)
Optimal speed37 mph42 mph47 mph
Starting effort48,676 lbf50,430 lbf47,832 lbf
Calculated Values
steam locomotive
freight
passenger
last changed: 01/2024
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