Union Pacific Class MK-1

Union Pacific (Oregon Short Line) #2013, Class Mk-1 in service at Pocatello, Idaho. May 1950.

Union Pacific Class MK-1
Status	Scrapped
Technical specifications
Type	Steam
Manufacturer(s)	Baldwin Locomotive Works
Build date	1910-1911
Rebuilder	Union Pacific Railroad.
Total produced	Eighty-Six
Classification	Mk-1 / Mk-57
Model	Harriman Common Standard Light Mikado
Wheel arrangement	2-8-2
Dia. of Drivers	57"
Weight	Varies
Rail gauge	4 Feet 8 1/2 Inches (Standard gauge)
Valve gear	Walschaerts
Valves	12"
Cylinders	Two 23 3/4" x 30" (Originally 25" x 30")
Boiler type	Radial-Stay
Boiler diameter	82" (First sheet)
Boiler pressure	190 Lbs. PSI. (As-built)
Superheater	Most rebuilt with Schmidt Type.
Firegrate area	70.4 Sq ft.
Fuel type	Coal or Oil
Fuel capacity	Varies (Due to different Tender styles)
Braking	One or Two No. 5 New York Types. (According to 1925 Diagrams)
Performance figures
Tractive effort	47,495 Lbs. TE.
Factor of adhesion	4.28 (Standard) 4.30 (Superheated)
Axle load	55,000 Lbs. (Maximum)
Service information
Operator(s)	Union Pacific Railroad, Oregon Short Line Railway, Oregon Railroad and Navigation Co.
Nickname(s)	"Mikado" "MacArthur"
Disposition details
Retired	December 1946 to October 1958.
Preserved	UP Mk-1 #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), Oregon Short Line Railroad (OSL) and Union Pacific Railroad (UP) in 1910 and 1911.

History

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, 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. 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.

Design

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. 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. 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. The brick arch was deeper than usual, again to suit lignite, and was supported by four arch tubes. The boiler, as built was originally Un superheated and had 495× 2" tubes. At the time this was the largest Mikado built, although not quite the most powerful or with the most weight on its drivers.

Superheating

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.

Oil firing

Some were later converted to oil firing.