Based on the success of its first two 4-8-2s (renumbered 4000 and 4001) the "Rock Island" ordered ten "Mountains" (road numbers 4002 through 4011) from ALCO in 1920. These Class M50 coal burning locomotives had 28 x 28 cylinders, 74" drivers, a boiler pressure of 200 psi, a tractive effort of 50,430 lbs and weighed 372,850 pounds.
In 1923, thirty more Class M50 locomotives (road numbers 4012 through 4041) were received from ALCO. This time, there was a mix of thirteen oil burners and seventeen coal burners. This batch was identical to the locomotives delivered in 1920 except that the coal burners weighed 367,850 lbs and the oil burners weighed 366,000 pounds.
Three more batches were received: five (road numbers 4042 through 4046) in 1926, ten (road numbers 4047 through 4056) in 1927 and five (road numbers 4057 through 4061) in 1929. These last twenty were built by ALCO and were similar to the others except for an increase in total weight. Four were oil burners and the balance burned coal.
A major rebuild occurred on numbers 4044 through 4061. These eighteen locomotives, designated as Class M50a, were given roller bearings on all axles and light weight running gear. The cylinder bore was reduced from 28" to 26" in a new cast cylinder and forward frame. The boiler pressure was raised to 220 psi, but with the reduced cylinder diameter there was a reduction in tractive effort, which was made up for in greater sustained steaming. These rebuilds were done in the Silvis Shops between 1940 and 1942.
Seven of the CRI&P's "Mountains" (road numbers 4006, 4007, 4015, 4022, 4023, 4028 & 4031) were sold to the St. Louis Southwestern Railroad in 1941. The balance were retired by 1953.
| Class M50 Locomotives | |||
|---|---|---|---|
| Road Numbers | Yr Blt | Locomotive Weight | Comment |
| 4000 - 4001 | 1913 | 333,100 lbs | all oil burners |
| 4002 - 4011 | 1920 | 372,950 lbs | all coal burners |
| 4012 - 4021 | 1923 | 367,850 lbs | 3 oil burners |
| 4022 - 4031 | 1923 | 366,000 lbs | all oil burners |
| 4032 - 4041 | 1923 | 367,850 lbs | all coal burners |
| 4042 - 4046 | 1926 | 365,000 lbs | 3 oil burners |
| 4047 - 4056 | 1927 | 370,500 lbs | 1 oil burner |
| 4057 - 4061 | 1929 | 378,000 lbs | all coal burners |
| Specifications | |||
|---|---|---|---|
| Class M50 | Class M50a | ||
| Coal Burners | Oil Burners | ||
| Wheel Arrangement: | 4-8-2 | 4-8-2 | 4-8-2 |
| Length: | 100' - 2" | 100' - 3" | 100' - 3" |
| Drivers: | 72" | 72" | 74" |
| Weight on Drivers: | 258,800 lbs | 254,000 lbs | see chart |
| Total Locomotive Weight: | 372,850 lbs | 366,000 lbs | see chart |
| Locomotive & Tender Weight: | 725,250 lbs | 714,750 lbs | see chart |
| Grate Area: | 63 sq ft | 63 sq ft | 63 sq ft |
| Cylinders: | (2) 28" dia. x 28" stroke | (2) 28" dia. x 28" stroke | (2) 26" dia. x 28" stroke |
| Boiler Pressure: | 200 psi | 200 psi | 220 psi |
| Tractive Effort: | 50,240 lbs | 50,240 lbs | 47,832 lbs |
| Tender Capacity: | 20,000 gals. of water and 20 tons of coal. | 20,000 gals. of water and 5,000 gals of oil. | 20,000 gals. of water and 20 tons of coal. |
| Class M50a Locomotives | |||
|---|---|---|---|
| Road Numbers | Fuel | Wt. on Drivers | Locomotive Weight |
| 4044-4046 | Coal | 253,500 lbs | 378,500 lbs |
| 4047 | Oil | 252,900 lbs | 372,600 lbs |
| 4048-4051 | Coal | 258,500 lbs | 376,500 lbs |
| 4052 | Coal | 259,250 lbs | 375,450 lbs |
| 4053-4054 | Oil | 253,450 lbs | 371,600 lbs |
| 4055 | Coal | 257,400 lbs | 372,900 lbs |
| 4056 | Oil | 251,500 lbs | 369,100 lbs |
| 4057-4061 | Coal | 253,500 lbs | 378,500 lbs |
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