Santa Fe 4-8-2 "Mountain" Locomotives of the USA

The Atchison, Topeka & Santa Fe Railway was experiencing the same problem with its "Pacifics" that many railroads were having. The "Pacifics" were not adequate for the longer passenger trains on many of its routes. Looking for more power, the AT&SF bought two 4-8-2 "Mountains" from the Baldwin Locomotive Works in 1918. One of them, road number 3700, was a coal burner and the other, road number 3701, burned oil. Before these two were delivered, another ten were ordered: five coal burners and five oil burners (road numbers 3702 through 3711). Between 1919 and 1924, the AT&SF would purchase another 39 "Mountains" (road numbers 3712 through 3750). Twenty of them were oil burners and nineteen were coal burners.

All 51 of these "Mountains" were built by the Baldwin Locomotive Works and were designated as Class 3700. They all had 28 x 28 cylinders, 69" drivers, a boiler pressure of 210 psi, a tractive effort of 56,788 lbs and weighed approximately 352,000 pounds.

The entire group of Class 3700 "Mountains" were retired and scrapped between 1950 and 1955.

Roster by Richard Duley

ClassRoad NumbersYear BuiltBuilder

Class Details by Steve Llanso of Sweat House Media

Class 3700 (Locobase 194)

Data from AT&SF 9 -1920 locomotive diagrams supplied in May 2005 and ATSF 1953 Locomotive Diagrams supplied in August 2013 by Allen Stanley from his extensive Raildata collection and Kansas Memory's holding of Santa Fe steam locomotive diagrams beginning at, last accessed 19 May 2013. See also "Mountain Type Locomotive for the Santa Fe", Railway Review, Volume 67 (16 November 1918), p. 697. "Three Types of Locomotives for the Atchison, Topeka & Santa Fe Railway System," Railway & Locomotive Engineering, (December 1919), pp. 363-365, and E D Worley, Iron Horses of the Santa Fe (Dallas: Southwest Railroad Historical Society, 1976), pp. 290-292.

All of Drawing Room Specification Book Number 61 of the DeGolyer Library's Baldwin collection is devoted to Santa Fe locomotive orders in the late 19teens and 1920s. The 3700 class specs are found in pages 225-338 and cover four batches; Volume 70, pages 490-711 contains the specs for the remaining batches.

(Many thanks to Chris Hohl for his 9 May 2015 email and spreadsheet laying out the variations among the batches with regard to heating surface areas, fuel, and tender capacities and weights.

Information about how many, when, and by whom this class was supplied from (accessed 6 Feb 2006), the Santa Fe All-Time Steam Roster maintained by Evan Werkema. Works numbers were:


May 48576, 48736


February 51261 in February 1919

March 51482, 51591-51592, 51627-51628

April 51694, 51739-51741


July 53467

August 53508-53509, 53560, 53578-53579, 53613

September 53686-53687, 53760


August 54983-54994

September 55044-55046


August 56866-56868

September 57004-57008


March 57699-57703

April 57755.

Firebox heating surface included 38 sq ft (3.5 sq m) of arch tubes and 89.8 sq ft (8.34 sq m) in the combustion chamber. 3722-3736 arrived with slight changes in heating surface areas that added up to 4,795 sq ft; superheater area was recalculated to 1,124 sq ft. 3721 was the first of the class to use a feed water heater, an Elesco E-1 model specified for that engine only in the 11 February 1920 order for road numbers 3717-3721.

The first two locomotives were fitted with Delta trailing trucks, but different outside radial valve gear systems. 3700 used Baker gear while 3701 was equipped with Walschaert gear. Because the designs were otherwise very similar, the valve timings and eccentric throw differences most likely illustrate design contrasts between Baker and Walschaert gear. The table just below shows some of these differences:


Engine Gear Valve travel Lap Clearance Throw

3700 Baker 6 5/8 1 1/16 3/16 20 1/4

3701 Walschaert 6 1/4 1 1/8 3/8 18 3/4

Possibly for reasons outlined in Locobase 5499--the 3160-class Mikados--none of the other 3700s would use Baker gear. Production engines rolled on Hodges trailing trucks.

Note the square cylinder measurements; the stroke is usually longer than the cylinder bore is wide. Piston valves measured 15" (381 mm) in diameter. Both of the prototypes had main driver bearings that were 12" in diameter and 20" long. Production engines used 12" x 12" bearings.

Chris Hohl's research on tender capacities and weight showed that all of the batches were delivered with tenders carrying 12,000 US gallons (45,420 litres) of water. Other than the 3700, whose tender held 17 tons of coal, capacities in the rest were either 16 tons of coal or oil amounting to 4,000 gallons (15,140 litres). His table of capacities and weights:

Road # Fuel fuel capacity (tons/gallons) Tender weight (lbs.)

3700 Coal 17 242,000

3701 Oil 4,000 242,000

3702-3706 Oil 4,000 242,000

3707-3711 Coal 16 242,000

3712-3716 Coal 16 233,720

3717-3721 Oil 4,000 242,000

3722-3728 Oil 4,000 242,000

3729-3736 Coal 16 233,720

3737-3739 Coal 16 233,720

3740-3744 Oil 4,000 242,000

3745-3750 Oil 4,000 236,600

3745-3750 Oil 4,000 12,000 236,600

The class was later retrofitted with Elesco feed water heaters and the first few with the Duplex automatic stoker delivered with the later batches.

Later members in the class built in 1924 ran at 220 lb psi and showed a slightly higher tractive effort.

Retired in 1950-1955.

Class 3737, 3740 (Locobase 16085)

Data from DeGolyer Library's Baldwin collection, Volume 70, pp 646+. See also E D Worley, Iron Horses of the Santa Fe (Dallas: Southwest Railroad Historical Society, 1976), pp. 290-292. Works numbers were 56278 in August 1923, 57004-57008 in September, 57699-57703 in March 1924, and 57755 in April.

(Many thanks to Chris Hohl for his 9 May 2015 email and spreadsheet laying out the variations among the batches with regard to heating surface areas, fuel, and tender capacities and weights.)

Locobase 194 tells much of the story of this Mountain design, the only class of 4-8-2s to operate on the Santa Fe. This entry shows the last twelve, which retained many of the changes in the firebox and the shortened engine wheelbase introduced in the two engines shown in Locobase 16084, but did not include the thermic syphons of the last two locomotives.

The third locomotive in that order and the last eleven in the class had firebox heating surface areas that included 33 sq ft (3.07 sq m) of arch tubes and 67 sq ft (6.22 sq m) in the combustion chamber. Like the 3738s, these engines mounted their Elesco feed water heater on the front deck and did not have the characteristic cylindrical tube bundle mounted over the smokebox.

E D Worley commented that the 3700s were limited by their driver diameters in the increasingly speedy express service. When they were replaced by the heroic 4-8-4 series that began with 3751, the 3700s pulled fast freights on the prairies and long troop trains during World War Two. "While their glamour [sic] was never again to arise to the heights of the time when they headed 22 sections of the 'California Limited' westbound and 23 more eastbound on the same day, the 3700's [sic] gave entirely creditable service to the end without fuss, feathers or excitement."

Class 3738 (Locobase 16084)

Data fromDeGolyer Library's Baldwin specifications collection, Volume 70, p 612+. See also See also E D Worley, Iron Horses of the Santa Fe (Dallas: Southwest Railroad Historical Society, 1976), pp. 290-292. Works numbers were 56866-56867 in August 1923.

(Many thanks to Chris Hohl for his 9 May 2015 email and spreadsheet laying out the variations among the batches with respect to heating surface areas and tender capacities. His tables were themselves useful and prompted further research on this single batch as well.)

Locobase 194 refers to the main sequence of Mountain batches supplied by Baldwin to the Santa Fe in the 1920s. This pair was delivered with thermic syphons, which replaced the arch tubes delivered with all of the other engines. Direct heating surface area now included 67 sq ft (6.22 sq m) of combustion chamber and 86 sq ft (7.99 sq m) from three Nicholson thermic syphons.

These engines, plus one more delivered without syphons, also arrived with Elesco feed water heaters mounted on the front deck.

Specifications by Steve Llanso of Sweat House Media
Class37003737, 37403738
Locobase ID194 16085 16084
RailroadSanta Fe (ATSF)Santa Fe (ATSF)Santa Fe (ATSF)
Number in Class491249
Road Numbers3700-3737, 3740-37503737, 3740-37503738-3739
Number Built491249
Valve GearWalschaertWalschaertWalschaert
Locomotive Length and Weight
Driver Wheelbase18'18'18'
Engine Wheelbase39.50'38.92'38.92'
Ratio of driving wheelbase to overall engine wheebase 0.46 0.46 0.46
Overall Wheelbase (engine & tender)76.80'76.80'76.78'
Axle Loading (Maximum Weight per Axle)60800 lbs61600 lbs62400 lbs
Weight on Drivers240640 lbs240900 lbs243700 lbs
Engine Weight355760 lbs353400 lbs365000 lbs
Tender Light Weight233720 lbs242400 lbs233720 lbs
Total Engine and Tender Weight589480 lbs595800 lbs598720 lbs
Tender Water Capacity12000 gals12000 gals12000 gals
Tender Fuel Capacity (oil/coal)16 tons400016 gals16 tons
Minimum weight of rail (calculated)100 lb/yard100 lb/yard102 lb/yard
Geometry Relating to Tractive Effort
Driver Diameter69"69"69"
Boiler Pressure200 psi200 psi200 psi
Cylinders (dia x stroke)28" x 28"28" x 28"28" x 28"
Tractive Effort54085 lbs54085 lbs54085 lbs
Factor of Adhesion (Weight on Drivers/Tractive Effort) 4.45 4.45 4.51
Heating Ability
Firebox Area373 sq. ft359 sq. ft408 sq. ft
Grate Area71.70 sq. ft71.70 sq. ft71.70 sq. ft
Evaporative Heating Surface4792 sq. ft4784 sq. ft4833 sq. ft
Superheating Surface1087 sq. ft1092 sq. ft1092 sq. ft
Combined Heating Surface5879 sq. ft5876 sq. ft5925 sq. ft
Evaporative Heating Surface/Cylinder Volume240.14239.74242.20
Computations Relating to Power Output (More Information)
Robert LeMassena's Power Computation143401434014340
Same as above plus superheater percentage169211706516921
Same as above but substitute firebox area for grate area880288544296288
Power L1189491891119250
Power MT694.40692.26696.58



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