Class MK - Spl (Locobase 6608) 
Data from 1936 Union Pacific locomotive diagram book supplied in May 2005 by Allen Stanley from his extensive collection.
Locobase reproduces this basic USRA light Mike design as modified by the Union Pacific to illustrate how the locomotives were updated during the next few decades.
Some of the class were fitted with Nicholson thermic syphons. The design retained 13.5 sq ft of arch tubes while the syphons themselves represented 78 sq ft of firebox heating surface. With other changes, the total increase in firebox heating surface totalled 57.5 sq ft or 20%. That was a healthy slug of new power achieved by concentrating on the part of the heat generator that would benefit most from a change.
Class MK 1/MK 2/MK-3 - superheated (Locobase 6607)
Data from 1936 Union Pacific locomotive diagram book supplied in May 2005 by Allen Stanley from his extensive collection.
This was the more powerful class of Mikados in its saturated state (Locobase 38). It wasn't long before the railroad was superheating those Mikes it hadn't bought with superheaters. The upgrade dried the steam, but otherwise little changed the overall balance.
Eight of the ten MK-3s were converted to oil burning; these had no grate area and no arch tubes and trailed tenders carrying 9,000-10,000 US gallons of water and 3,750 US gallons of oil.
Class MK-1 (Locobase 1405)
Data from UP's 1918 Locomotive diagram book supplied in May 2005 by Allen Stanley from his extensive collection. See also "Simple 2-8-2 for Oregon Railroad Navigation Co.," Railroad and Locomotive Engineering, Vol 23, No 10 (October 1910), pp. 417-418; and "Powerful Lignite Burner of the Mikado Type", American Engineer and Railroad Journal (October 1910), pp. 404-406
MK-1 and MK-2 -- all Baldwin-built -- represent the first of two principal Mikado designs that EH Harriman settled on to standardize his locomotive stud. The first Harriman 2-8-2 was OR&N's 2100. Later engines had squarer cylinder dimensions (see Locobase 38).
But the big difference between this Mike design and several others that were entering service at the same time was its provision for burning low-calorie lignite coal. As a result, the a radial-stay boiler was substituted for the usual crown-bar boiler. The AERJ article comments that the size of the boiler is such that were it to use a good-quality bituminous coal, the engine would be considered "over-boilered." Incorporating the experience of burning lignite in Chicago, Burlington & Quincy locomotives, the grate had many narrow openings and a high-mounted firebrick arch to soften the pull of the draft on the coal bed. Smokebox differences included a large area, fine netting to catch sparks, and a low-mounted nozzle and bell-mouthed extension on the stack.
AERJ notes that in all other respects, this class of 2-8-2s followed Associated Lines designs, including 25" pistons bushed to 23 3/4", 12" piston valves with 1/4" lead.
MK-1s went to UP (30 in 1911), Oregon Short Line (15 in 1911) , and Oregon Washington Railroad & Navigation (41 1910 & 1911). MK-2s followed for UP (20 in 1912), Oregon Short Line (20 in 1912), and OWR&N (25 in 1912-1913). Works numbers were
1910
July 34978 (OWR&N)
1911
March 36227-36236 (UP), 36237-36246, 36322-36329 (OWR&N), 36277-36280 (OSL)
April 36281, 36362-36371 (OSL), 36330-36351 (OWR&N), 36352-36361, 36448-36451 (UP)
May 36517-36521 (UP)
September 38293-38294, 38328-38334, 38356-38361 (OWR&N).
Retirements began in 1947.
Class MK-1 - superheated (Locobase 6605)
Data from 1936 Union Pacific locomotive diagram book supplied in May 2005 by Allen Stanley from his extensive collection. For new construction, works numbers were 38362-38363, 38385-38386 in September 1912; 38421-38425, 38490-38500 in October 1912 (UP); 38543-38562 in October (Oregon Short Line).
After 80 of the saturated-boiler MK-1s were delivered to the various Associated Lines in 1910-1911 (Locobase 1405), Baldwin began producing superheated versions. Superheating the MK-1 Mikado class seems to have consumed a surprising amount of space. From 495 2" tubes in the saturated boiler, the new design traded 220 tubes for 36 superheater flues. One could argue that the original design was over-endowed. The firebox heating surface area included 32.2 sq ft of "fire brick" arch tubes.
Most of the MK-1s and all of the sextet of OWRRN MK-2s were so modified. Some had had their arch tubes deleted by 1946 during their conversions to oil-firing. Their tenders carried 3,750 US gallons of oil.
Class MK-2 (Locobase 14089)
Data from DeGolyer, Vol 46, 51. Works numbers were 39905-39914 in May 1913.
This class had only detail differences from the Mk1s shown in Locobase 6605. The first 10 were renumbered in 1915 when the OWR & N was integrated into the UP's motive power roster.
All ten engines served the UP throughout World War II. The 2163 was the first to be retired in May 1947 while 2157 was the last more than a decade later in January 1958.
Class MK-3 (Locobase 38)
Data from UP 5-1918 Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive collection. Works numbers were 36511-36515, 36562-36563, 36573-36575 in May 1911.
Relatively high-boilered type that differed from the earlier MK-1/MK-2 (Locobase 1405) in having squarer cylinder dimensions and taller drivers; one of two principal designs that EH Harriman settled on to standardize his Associated Lines locomotive stud.
Locobase 6607 describes the superheated upgrade that was very shortly applied to this class.
Retirements began in 1947.
Class MK-4 (Locobase 14239)
Data from DeGolyer, Vol 46, pp. 81+. Works numbers were 36511-36515, 36562-36575 in May 1911.
These were the first Baldwin Mikes for the UP that were delivered with superheaters in what was now the standard 2-8-2 design for that road.
All of the Mk-4s remained with the UP. 2200 was the first to be retired in October 1947. 2206 outlasted almost all of the other 2-8-2s, being retired only in December 1957.
Class MK-5 (Locobase 14237)
Data from DeGolyer, Vol 46, pp. 81+. Works numbers were 39587-39607, 39674-39677 in April 1913.
These were the first Baldwin Mikes for the UP that were delivered with superheaters in what was now the standard 2-8-2 design for that road. Four--2224-2225, 2228, and 2233--were sold to subsidiary Oregon Short Line in 1923 as that road's 2520-2523.
Of the Mk-4s that remained with the UP, 2237 was the first to be retired in March 1947. The last to be delivered--2244-- was the last retired in May 1956.
Class MK-5 (Locobase 14238)
Data from DeGolyer, Vol 46, pp. 81+. Works numbers were 39587-39607, 39674-39677 in April 1913.
These were the first Baldwin Mikes for the UP that were delivered with superheaters in what was now the standard 2-8-2 design for that road. Four--2224-2225, 2228, and 2233--were sold to subsidiary Oregon Short Line in 1923 as that road's 2520-2523.
Of the Mk-4s that remained with the UP, 2237 was the first to be retired in March 1947. The last to be delivered--2244-- was the last retired in May 1956.
Class MK-6 (Locobase 14243)
Data from DeGolyer, Vol 46, pp. 81+. Works numbers were 36511-36515, 36562-36575 in May 1911.
Lima Locomotive Works' first Mikados for the Union Pacific repeated the power dimensions, weights, and areas of the Baldwins that had arrived in 1912 (Locobase 14239). Lima's fifteen were supplemented by ten delivered to subsidiary Los Angeles & Salt Lake. Alco built 9, Baldwin 1.
Class MK-7/MK- 8/MK-9 (Locobase 11022)
Data from Record of Recent Construction #98 (Baldwin Locomotive Works, 1920), p. 19. See also DeGolyer, Vol 56, pp. 217+.
1917
June 45765-45768, 45830, 45841-45843; July 45927-45929, 45987-45988, 45999-46000; August 46155, 46161, 46172, 46212
1918
March 48080, 48143; April 48290-48293; May 48523, 48572, 48677-48678, 48734, 48777; June 48894, 48954, 48996
This class was delivered with superheaters. Nearly square cylinders received their steam through 15" (381 mm) piston valves.
With few exceptions, the thirty engines operated on the UP until the 1950s. In 1923, however, five were sold to the UP's Oregon Short Line subsidiary; 2266, 2268, 2275, 2277-2278 became 2528-2532.
The Oregon Washington Railroad & Navigation sold 2260-2261 to the Oregon & Northwestern in February 1947.
