Milwaukee Road 4-6-4 "Hudson" Locomotives in the USA

The Chicago, Milwaukee, St. Paul & Pacific Railroad bought 22 Hudsons from the Baldwin Locomotive Works, in the early 1930s. 14 Class F-6 (road numbers 6400 through 6413) were delivered in 1930 and eight Class F-6a (road numbers 6414 through 6421) arrived in 1931.

In 1938, CMStP&P ordered six 4-6-4, Class F-7 Hudsons from the American Locomotive Company and assigned them road numbers 100 through 105. These locomotives were styled by the noted designer Otto Kuhler and were used to lead the "Twin Cities Hiawatha". They reduced the running time for the 412 mile trip from the Twin Cities to Chicago to 6.5 hours.


ClassQtyRoad NumbersYear BuiltBuilder

Class Details by Steve Llanso of Sweat House Media

Class F-7 (Locobase 183)

Data from MILWRD 1945 Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange collection. Thanks to John Werth's 25 July 2012 email for alerting me to a low engine weight figure. (Many thanks to Chris Hohl for his 22 September 2017 email reporting unlikely boiler pressure values for 177 entries. A Locobase macro caused the error.) Works numbers were 69064-69069 in September 1938.

Firebox heating surface included 110 sq ft in three thermic syphons and arch tubes. These engines replaced the A-class 4-4-2s in the Chicago-Minneapolis "Hiawatha" runs and the last 4-6-4s built for a US railroad.. They were similar to the C&NW's E-4s shown in Locobase 180 (including boiler pressure, driver diameter, combined evaporative and superheated surface areas) and were built by the same builder.

The F-7s had smaller-diameter cylinders supplied by 12" (305 mm) diameter piston valves. Even with a 1" (25.4 mm) longer stroke, F-7s presented a 9.4% decrease in volume per cylinder, and a 10% smaller grate. Also, the F7s had greater volume in their tubes and flues and the ratio of tubes to flues was much closer to the average. By 1945, EHS had been reduced slightly 4,133.5 sq ft (348 sq m) with the substitution of American Arch Company circulators totalling 77.5 sq ft (7.2 sq m) for the syphons.

As it was late-1930s superpower, the design featured all the typical accessories and enhancements including roller bearings all around (Timken on the engine truck and drivers, ASF on the trailing truck under the firebox and on all tender axles), Boxpok drivers, and multiple exhaust nozzle

Jim Scribbins (in Drury, 1993) notes that the design was really only suitable for the elite passenger service. Once the Milwaukee dieselized the Hiawathas, the F-7s were relegated to the Arrow (Chicago-Omaha) and Chicago-Milwaukee service. Other locomotives could perform as well in this service at less cost, however, so the F-7s were the first Hudsons to be scrapped in 1949-1951.

Class F6/F6a (Locobase 182)

Data from MILWRD 1945 Locomotive diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange collection. See also DeGolyer, Volume 82, pp. 205+ and V E Rennix, "High Mileage Performance on the Milwaukee Road", Baldwin Locomotives (1934) found in [], last accessed 5 January 2013.(Many thanks to Chris Hohl for his 22 September 2017 email reporting unlikely boiler pressure values for 177 entries. A Locobase macro caused the error.) Works numbers were 61135-61148 in November 1929, 61655-61662 in August 1931.

Known on the CMStP&P as a "Baltic". The differences between the two classes were the broken running board line over an air reservoir on the first 14 (F6) and the straight running board of the last 8 (F6a). Data similar for both, but some heating surface areas (F6a in the specifications) are slightly different. The specs for the first fourteen called for a maximum weight on the drivers of 189,000 lb (85,729 kg), but the diagram book shows that, at least in later years, actual weight exceeded desired weight by almost four short tons.

Firebox heating surface included 14 sq ft (1.3 sq m) in arch tubes, 72 sq ft (6.7 sq ft) in the combustion chamber, and 90 sq ft (8.35 sq m) in three Nicholson thermic syphons (two in the firebox, one in the combustion chamber). They had 14" (356 mm) piston valves, Coffin feed water heaters, mechanical stokers, and mechanical and pressure lubrication.

A large example of American orthodoxy in steam passenger locomotives, an engine of this class set a flying-average mark of 92.3 mph (148.5 km/h) over 65.6 miles (105.6 km) of level track in 1934. Another completed 10 round trips between Minneapolis and Harlowton, Mont (918 miles) in 30 days with no days out for servicing. The regular consist was 9-10 cars, but 11-14 cars wasn't unusual, and 15 or more might be seen occasionally.

Tests showed an average of 6.4 lb (2.90 kg) of water evaporated per pound of coal fired and coal consumption per thousand gross ton-miles between 90.32 to 123.2 lb (40.97 to 55.88 kg). Such rates indicate the hard pressing that passenger engines received to tick off mile after mile of high-speed running - lots of wide-open throttle and late valve cut-offs.

A table in the Railway Age's 23 December 1933 story on this class (p. 876) provides a snapshot of what a large American locomotive could require in replenishment during one trip. Given that the locomotive was full of water and coal when leaving Minneapolis, it would stop nine times for water and need most of a tenderful upon arriving in Harlowton, Mont. Five coal stops - most of which were not coincidental with the water stops - would take the engine to Harlowton with about 8 tons left over. In addition, the crew would clean the fire twice and dump the ash pan five times, lube the main pins five times and all other pins twice.

Ed Kehm emailed Locobase in January 2013 to point out that the F6s were delivered with 79" (2,007 mm) drivers. By the time VE Rennix of Baldwin's Chicago office detailed the F6's performance in 1934, however, his list of specifications included 80" drivers. The difference lay in the use of 4" (102 mm) tires on the wheels. The difference in starting tractive effort was less than 600 lb (272 kg or 2.67 kN) .

Principal Dimensions by Steve Llanso of Sweat House Media
Locobase ID183 182
RailroadMilwaukee Road (CMStP&P)Milwaukee Road (CMStP&P)
Number in Class622
Road Numbers100-1056400-6421
Number Built622
Valve GearWalschaertBaker
Locomotive Length and Weight
Driver Wheelbase (ft / m)14.67 / 4.4714 / 4.27
Engine Wheelbase (ft / m)42.33 / 12.9040.75 / 12.42
Ratio of driving wheelbase to overall engine wheelbase 0.35 0.34
Overall Wheelbase (engine & tender) (ft / m)89.83 / 27.3881.62 / 24.88
Axle Loading (Maximum Weight per Axle) (lbs / kg)
Weight on Drivers (lbs / kg)216,000 / 97,976196,550 / 89,154
Engine Weight (lbs / kg)415,000 / 188,241380,220 / 172,465
Tender Loaded Weight (lbs / kg)375,000 / 170,097287,780 / 130,535
Total Engine and Tender Weight (lbs / kg)790,000 / 358,338668,000 / 303,000
Tender Water Capacity (gals / ML)20,000 / 75.7615,000 / 56.82
Tender Fuel Capacity (oil/coal) (gals/tons / Liters/MT)25 / 2320 / 18
Minimum weight of rail (calculated) (lb/yd / kg/m)120 / 60109 / 54.50
Geometry Relating to Tractive Effort
Driver Diameter (in / mm)84 / 213480 / 2032
Boiler Pressure (psi / kPa)300 / 20.70225 / 15.50
High Pressure Cylinders (dia x stroke) (in / mm)23.5" x 30" / 597x76226" x 28" / 660x711
Tractive Effort (lbs / kg)50,294 / 22813.0045,250 / 20525.08
Factor of Adhesion (Weight on Drivers/Tractive Effort) 4.29 4.34
Heating Ability
Tubes (number - dia) (in / mm)60 - 2.25" / 5758 - 2.25" / 57
Flues (number - dia) (in / mm)164 - 3.75" / 95182 - 3.5" / 89
Flue/Tube length (ft / m)19 / 5.7919 / 5.79
Firebox Area (sq ft / m2)458 / 42.55411 / 38.18
Grate Area (sq ft / m2)96.50 / 8.9780 / 7.43
Evaporative Heating Surface (sq ft / m2)4166 / 387.034205 / 390.65
Superheating Surface (sq ft / m2)1695 / 157.471815 / 168.62
Combined Heating Surface (sq ft / m2)5861 / 544.506020 / 559.27
Evaporative Heating Surface/Cylinder Volume276.63244.33
Computations Relating to Power Output (More Information)
Robert LeMassena's Power Computation28,95018,000
Same as above plus superheater percentage37,34623,400
Same as above but substitute firebox area for grate area177,246120,218
Power L160,54939,387
Power MT1853.991325.36

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Wes Barris