Union Pacific class MK-1

Union Pacific class MK-1
2111 in the snow in Idaho in the 1930s
Type and origin Power type steam [1] Builder Baldwin Locomotive Works Build date 1910-1911 Total produced 86
Specifications Configuration: ​  • Whyte 2-8-2 Gauge 4 ft 8+1⁄2 in (1,435 mm) Leading dia. 30½" Driver dia. 57" Trailing dia. 36" Wheelbase: ​  • Engine 34 ft 8 in  • Drivers 16 ft  • incl. tender 64 ft 7 in Axle load: ​  • Leading 24,100 lb  • Trailing 34,550 lb Adhesive weight 204,450 lb Loco weight 263,100 lb Tender type Vanderbilt tender in later batches[2] Fuel type lignite Fuel capacity 10 tons Water cap. 9,000 US gallons Firebox: ​  • Grate area 70 sq ft Boiler: ​  • Tube plates 20 ft 6 in  • Small tubes 495× 2" Boiler pressure 180 psi Heating surface: ​  • Firebox 267 sq ft  • Tubes 5,292 sq ft  • Total surface 5,559 sq ft Cylinders 2 Cylinder size 23¾" × 30" Valve gear Walschaerts
Performance figures Tractive effort 45,300 lbf
Career Class UP: MK-1 Numbers 1900-1929, 2000-2014, 2100-2140[3] Preserved No. 2005 Disposition One preserved, remainder scrapped

The Union Pacific class MK-1 was a class of 2-8-2 'Mikado' type steam locomotives that were built by the Baldwin Locomotive Works originally for the Oregon Railroad and Navigation Company (OR&N),^[4] Oregon Short Line Railroad (OSL)^[2] and Union Pacific Railroad (UP) in 1910 and 1911.^[2]

In 1910, the Oregon-Washington Railroad and Navigation Company (OWR&N) purchased a 2-8-2 'Mikado' locomotive from the Baldwin Locomotive Works and had this single locomotive numbered 440,^[4] this locomotive would be renumbered as 500 in 1911, and renumbered to 2100 in 1915.

In 1911, the OWR&N would order another 40 of these locomotives from Baldwin where they were originally numbered as 501–540, but renumbered as 2101–2140. The Oregon Short Line Railroad (OSL) would order 15 locomotives from Baldwin and numbered them as 1100–1114, but renumbering them to 2000–2014 in 1915.^[2] The Union Pacific Railroad (UP) would order 30 locomotives from Baldwin where they numbered them as 500–529, renumbering them to 1900–1929 in 1915

OSL No. 2000 was sold to the Spokane International Railroad (SI) in 1947.^[5]

The locomotives were designed from the outset to burn low-quality lignite coal as a fuel. In general they followed typical practice of the day for Baldwin designs and locomotives for Harriman's Associated Lines, but with some changes to the boiler and firebox.^[1] The boiler was cylindrical and the firebox was stayed with 400 radial rod stays, rather than girder crown stays. Given the poor quality of the fuel, a larger boiler was needed than usual for the power of the locomotive. The heating surface of 5,559 sq ft considerably exceeded that of the nearest comparable design, with 4,466.^[i] The grate bars were arranged as large cast grate sections spanning half the width of the firebox and arranged for rocking to clear ash. Airflow was provided by an unusually great number of narrow slots, to suit lignite.^[7] The brick arch was deeper than usual, again to suit lignite, and was supported by four arch tubes. The boiler, as built, was unsuperheated and had 495× 2" tubes.^[1]

At the time this was the largest Mikado built, although not quite the most powerful or with the most weight on its drivers.^[1][6]

In 1912, the boiler was rebuilt to provide superheating. Half of the tubes were removed in favour of 36 larger flues containing superheater elements. Total heating area was reduced to 5,118 sq ft, although still large in its class.^[i][2]

Some were later converted to oil firing.^[2]

Only one member of the class is in preservation, No. 2005, which is on static display at Ross Park based in Pocatello, Idaho.^[2]