The reference for locomotives and railcars


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Pennsylvania classes M1 and M1a
United States | 1923 | 301 produced
M-1b No. 6755 preserved in not very good condition
M-1b No. 6755 preserved in not very good condition
James St. John / Pennsylvania Railroad # 6755 steam locomotive (M1b 4-8-2) 3
No. 6926 in May 1937 in Altoona, Pennsylvania
No. 6926 in May 1937 in Altoona, Pennsylvania
collection Taylor Rush

The PRR's first Mountain was created in 1923 by placing the boiler of the Class I1s Decapod on a new frame. After testing the prototype, 175 production locomotives were built by Baldwin and 25 by Lima. In 1930, 50, 25 and 25 locomotives from Baldwin, from Altoona and from Lima followed of the M1a, which had been improved in details. Not only could they pull heavy passenger trains at high speeds, they were also not overwhelmed by freight trains with more than 100 cars and 4,000 short tons in the mountains. 40 M1a were converted to the M1b in 1946 by increasing the boiler pressure by 20 psi and increasing the size of the firebox

Built1923, 19261930
ManufacturerAltoona, Baldwin, Lima
Axle config4-8-2 (Mountain) 
Gauge4 ft 8 1/2 in (Standard gauge)
Dimensions and Weights
Length87 ft 7 5/8 in
Wheelbase41 ft 0 1/2 in41 ft 10 in
Fixed wheelbase18 ft 10 in
Total wheelbase79 ft 4 in
Service weight385,850 lbs390,000 lbs
Adhesive weight267,000 lbs271,000 lbs
Total weight606,900 lbs768,360 lbs
Axle load72,500 lbs70,000 lbs
Water capacity11,980 us gal22,020 us gal
Fuel capacity37,000 lbs (coal)63,000 lbs (coal)
Power sourcesteam
Estimated power3,500 hp (2,610 kW)
Optimal speed35 mph32 mph
Starting effort64,547 lbf69,417 lbf
Power Plant
Driver diameter72 in
Boiler pressure250 psi
Expansion typesimple
Cylinderstwo, 27 x 30 intwo, 28 x 30 in
Grate area69.9 sq ft
Firebox area397 sq ft
Tube heating area4,303 sq ft4,305 sq ft
Evaporative heating area4,700 sq ft4,702 sq ft
Superheater area1,632 sq ft1,630 sq ft
Total heating area6,332 sq ft
Calculated Values
steam locomotive
J.T. Wallis
last changed: 03/2023

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