The reference for locomotives and railcars


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Chicago, Milwaukee, St. Paul & Pacific (Milwaukee Road) class A2
United States | 1901 | 43 produced
No. 919 on a Baldwin works photo
No. 919 on a Baldwin works photo

The Class A2 designated the first Atlantics on the Milwaukee Road to have a Vauclain-type compound engine. This means that there was a high and a low pressure cylinder on each side, which were connected to a common crosshead. The trailing axle had special inside bearings developed by J.H. DeVoy that were part of a large single casting.

Nine A2 and 15 A2a were built by Baldwin in 1901 and 1902 respectively. The five A2b had drivers whose diameter had been increased to 85 inches. While these were built in the railroad's own workshops in Milwaukee, the twelve A2cs were built again at Baldwin. These were all scrapped between 1927 and 1930.

An exception were two locomotives that were built in 1907. These had what was known in the USA as a “balanced compound” drive, which had separate inside and outside cylinders. These locomotives were rebuilt in 1938 to class A-4a with simple expansion and remained in service until 1951.

VariantA2A2aA2cA2 balanced compound
Axle config4-4-2 (Atlantic) 
Gauge4 ft 8 1/2 in (Standard gauge)
Dimensions and Weights
Wheelbase27 ft 11 1/2 in29 ft 2 1/2 in32 ft 2 in
Fixed wheelbase7 ft 3 in7 ft 4 in7 ft 6 in
Total wheelbase56 ft 5 1/2 in61 ft 8 in53 ft 3 in
Service weight181,535 lbs177,470 lbs210,400 lbs195,000 lbs
Adhesive weight100,335 lbs92,450 lbs108,750 lbs106,000 lbs
Total weight321,000 lbs310,000 lbs343,000 lbs329,000 lbs
Axle load50,168 lbs46,225 lbs54,375 lbs53,000 lbs
Water capacity7,000 us gal
Fuel capacity18,000 lbs (coal)20,000 lbs (coal)
Power sourcesteam
Estimated power1,600 hp (1,193 kW)1,525 hp (1,137 kW)1,750 hp (1,305 kW)
Optimal speed54 mph55 mph48 mph55 mph
Starting effort18,750 lbf18,694 lbf20,382 lbf
with start valve22,500 lbf22,433 lbf24,458 lbf
Power Plant
Driver diameter84 in84.3 in85 in
Boiler pressure200 psi220 psi
Expansion typecompound
Cylindersfour, HP: 15 x 28 in
and LP: 25 x 28 in
Grate area46.7 sq ft45 sq ft45.8 sq ft
Firebox area183.3 sq ft173.3 sq ft214.3 sq ft183 sq ft
Tube heating area3,008.7 sq ft2,973.7 sq ft3,015 sq ft
Evaporative heating area3,192 sq ft3,182 sq ft3,188 sq ft3,198 sq ft
Total heating area3,192 sq ft3,182 sq ft3,188 sq ft3,198 sq ft
Calculated Values
steam locomotive
Vauclain compound
Von Borries compound
last changed: 05/2023

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