Union Pacific 4-8-2 "Mountain" Locomotives in the USA

The Union Pacific Railroad took delivery of forty 4-8-2s (road numbers 7000 through 7039) from the American Locomotive Company in 1922. These Class MT-1 "Mountains" had 29 x 28 cylinders, 73" drivers, a boiler pressure of 200 psi, a tractive effort of 54,838 lbs and each weighed 348,000 pounds. A year later, fifteen more (road numbers 7850 through 7864) were added to the roster. These last fifteen, ALCO built, Class MT-1s were similar to the ones delivered in 1922 except they weighed 345,000 pounds.

In 1924, five more "Mountains" came from ALCO. These locomotives were designated as Class MT-2 and were assigned road numbers 7865 through 7869. They were oil burners and had 29 x 28 cylinders, 73" drivers, a boiler pressure of 200 psi which resulted in a tractive effort of 54,838 pounds. The combined weight of the Class MT-2 locomotive and its tender was 634,740 lbs with a full load of 15,000 gallons of water and 5,480 gallons of oil.

All of the Union Pacific "Mountains" were scrapped by 1956.


ClassRoad NumbersYear BuiltBuilder

Class Details by Steve Llanso of Sweat House Media

Class MT-1/MT-2 (Locobase 230)

Data from 1936 Union Pacific locomotive diagram book supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. See also Railway Journal, Volume 28, No 7 (July 1922), pp. 14-15, "Boiler of Union Pacific Mountain Type Locomotive," Boiler Maker (August 1922), pp. 217-222; and "A New Mountain for the Union Pacific," Railway Age, Volume 72, No 23 (10 June 1922), pp. 1325-1329 and Volume 73, No 16 (14 October 1922), pp. 687-689. See Locobase 6082 for a full description of the Young valve gear. Locobase thanks Gordon McCulloh for his 3 Feb 2012 email asking about the TTTs, which induced a rewrite of several entries.

These were the only Mountains the Union Pacific ran. When RJ's July 1922 issue featured this greyhound, the author commented on the "lightness of parts" that went into the design. He noted that the 2-8-2s then in service were not equal to the task of maintaining an average speed of 37 mph (60 kph) over the 484 miles (779 km) between Ogden, Utah and Cheyenne, Wyoming; their drivers were simply too small to meet the higher speed requirements.

In addition, the author said, "It may not be generally known that there are grades west of Cheyenne in the Wyoming division of long distances running as high as 1.55 per cent on the westbound, and 1.14 per cent grades on the eastbound operation. Six through passenger trains pass over this region daily"

Then RJ's reporter noted the catch in using a 4-8-2 layout with taller drivers: "The only difficulty in introducing the mountain type of locomotive was in the recognized fact that the weight should not exceed 345.000 Ibs.[156,490 kg], for while the railroad, as is well known, is double tracked and of the most substantial construction, the numerous bridges, viaducts and other structures are not calculated to meet the requirements of heavier motive power than the weight referred to."

The result was a success: "[7000] has not been approached in point of design where boiler capacity in relation to weight is considered, and has only been made possible by a close comparison with other types where weights are known and by thorough tests of individual parts and material in order to ascertain what was possible as the looked for accomplishment, looking towards a general introduction of the mountain type of locomotive, if the success of the experiment should prove beyond controversy."

Railway Age's 14 October 1922 summary of the 7000-class's early performance described a "splendid impression among the crews who have operated it." Reasons included "its most riding qualities, particularly around curves, as well as its steaming qualities and ability easily to maintain schedules." An additional final comment noted "The well balanced proportions on the machinery is attessted to by the ability of the locomotive to operate continuously on runs varying from 500 to 700 miles [805-1,127 km] in length."

Indeed, Alco's boast in a November 1922 advertisement noted that the lead engine took Train #4, consistion of 11 cars aggregating a trailing load of 816 tons, up a 0.82% grade at a 50 mph (81 kph) sustained speed. At that time, the locomotive was developing 3,500 indicated horse power.

Firebox heating surface included 33.5 sq ft (3.1 sq m) of arch tubes as well as the surface area of the combustion chamber. Note the "oversquare" (cylinder diameter greater than stroke).

RJ also commented on the choice of Young valve gear for these big engines: "Mechanical officials of the Union Pacific Railroad are also placing considerable dependence on the Young valve gear as a means for increasing capacity of this locomotive on hard pulls. The ability of the Young valve gear to give a high mean effective cylinder pressure due indirectly to its long travel, is well known. In full gear, the valves on this locomotive, which are 14 inches in diameter, have a travel of 9 in." (9 inches of valve travel was indeed long for that day or any other steam era.)

A Trainorders forum thread headed UP Steam Question and started by "yardclerk" included a 6 December 2006 entry, time-stamped 4:32 AM ([],1299181, last accessed 8 February 2012) in which "4-12-2" added this interesting tidbit: "By the time Union Pacific ordered the first of their 4-8-2's for 1922 delivery the Young gear was being marketed by Pyle-National, a company most think was involved strictly in electric appliances. I remember my surprise when I first learned this. I don't know if this was P-N's only venture into the marketing of locomotive appliances outside their huge array of electrical products, but I do believe it represented a major departure for P-N and I think speaks volumes about the promise held in the Young design, otherwise I don't believe P-N would have "taken the chance" on this product."

The RJ reporter noted also the steam-passage design: "Morever, as will be noted from the drawings, the steam passages in the cylinders are amply proportioned, being cored in accordance with the Cole patent."

In the 1930s, the class was modified with Walschaert valve gear and one-piece cast-steel frames. Locomotives 7850-7864 were leased to the Los Angeles & Salt Lake.

The diagrams show that many were later fitted with a streamliner casing and bullet nosecap. They also reveal the 16" (406 mm) piston valves that went in at that time. Wes Barris's steamlocomotive.com site ([], last accessed 27 February 2010 ) shows a more extensive streamlining of 7002. The purpose of streamlining 7002 (and Pacific 2906, described in Locobase 6612) was to substitute for the early diesels when they were unable to pull the heavy, all-Pullman Forty-Niner trains 5 times a month between Chicago and San Francisco in 1938-1941 to help celebrate the Golden Gate Exposition. Like the 2906, the UP fitted Timken roller bearings on every engine axle and in the rods as well. Weight grew to 257,500 lb (116,880 kg) on the drivers and 382,500 lb (173,499 kg) for the engine overall.

Like that of the 2906, the casing adopted fell short of aesthetic triumph to perhaps a greater degree than the installation on the 2906. The most dominant color was chocolate brown (called Leaf Brown by the UP), accented by the Armour Yellow stripe on the bluntly curved nose (it had yellow whiskers as well), Scarlet trim above yellow on the valences that underlay the running boards, lazy-oval cab windows, and the trademark yellow-with-red stripe tender. The overall effect was to reduce the mighty Mountain and its six-axle tender to an American Flyer nightmare.

Principal Dimensions by Steve Llanso of Sweat House Media

Locobase ID230
RailroadUnion Pacific (UP)
Number in Class65
Road Numbers7000-7039, 7850-69
Number Built65
Valve GearYoung
Locomotive Length and Weight
Driver Wheelbase (ft / m)19.60 / 5.97
Engine Wheelbase (ft / m)41.25 / 12.57
Ratio of driving wheelbase to overall engine wheelbase 0.48
Overall Wheelbase (engine & tender) (ft / m)
Axle Loading (Maximum Weight per Axle) (lbs / kg)58,290 / 26,440
Weight on Drivers (lbs / kg)233,060 / 105,714
Engine Weight (lbs / kg)350,250 / 158,871
Tender Loaded Weight (lbs / kg)237,500 / 107,728
Total Engine and Tender Weight (lbs / kg)587,750 / 266,599
Tender Water Capacity (gals / ML)12,000 / 45.45
Tender Fuel Capacity (oil/coal) (gals/tons / Liters/MT)20 / 18
Minimum weight of rail (calculated) (lb/yd / kg/m)97 / 48.50
Geometry Relating to Tractive Effort
Driver Diameter (in / mm)73 / 1854
Boiler Pressure (psi / kPa)200 / 13.80
High Pressure Cylinders (dia x stroke) (in / mm)29" x 28" / 737x711
Tractive Effort (lbs / kg)54,838 / 24874.13
Factor of Adhesion (Weight on Drivers/Tractive Effort) 4.25
Heating Ability
Tubes (number - dia) (in / mm)239 - 2.25" / 57
Flues (number - dia) (in / mm)48 - 5.5" / 140
Flue/Tube length (ft / m)22 / 6.71
Firebox Area (sq ft / m2)382 / 35.49
Grate Area (sq ft / m2)84 / 7.80
Evaporative Heating Surface (sq ft / m2)4974 / 462.10
Superheating Surface (sq ft / m2)1242 / 115.38
Combined Heating Surface (sq ft / m2)6216 / 577.48
Evaporative Heating Surface/Cylinder Volume232.32
Computations Relating to Power Output (More Information)
Robert LeMassena's Power Computation16,800
Same as above plus superheater percentage20,160
Same as above but substitute firebox area for grate area91,680
Power L120,528
Power MT776.73

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Wes Barris