Missouri Pacific 4-6-2 "Pacific" Locomotives in the USA

Class 6000 (Locobase 152)

Data from tables in 1930 Locomotive Cyclopedia. (Thanks to Chris Hohl for the valve gear ID and for pointing out the mixed valve-gear setup, which is described in Alfred Bruce, The Steam Locomotive in America (New York: Bonanza Books, 1952), pp. 201-201, 210; his 2018 email supplied the correct firebox heating surface area and loaded tender weight. Works number was 66026 in January 1925.

Firebox had combustion chamber. One of three three-cylinder Pacific locomotive designs built by Alco in 1925. (The others were Rock Island's P-46 class-Locobase 142-and L&N's #295--Locobase 149.) The MP was the heaviest, had more small tubes, and had a slightly higher superheat/CHS ratio. It also had by far the biggest tender.

As noted in the entry on MoPac's three-cylinder 2-8-2 (Locobase 22), trials showed low fuel consumption and impressive tractive effort. Bruce's diagram shows that like the other Alco three-cylinder locomotives, all three cylinders were line abreast and roughly equally spaced. But the two right-hand piston valves were closely spaced over the right-hand cylinder and linked together by a solid arm that pivoted around its center, thus actuating first the central valve, then the outside right valve.

In service, however, the maintenance demands of that inside third cylinder outweighed any performance benefit. For a long time, 6000 sat idle, then was rebuilt as essentially a new two-cylinder engine; see Locobase 9394.

Class 6001 (Locobase 9394)

Data from MP7 - 1956 Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. (Thanks to Chris Hohl for his 28 October 2019 email providing corrections to all three wheelbases and loaded tender weight and water capacity.)

Locobase 152 shows the original locomotive 6000 as it was delivered in 1925 with its three-cylinder layout. As noted, the three-cylinder arrangement's additional power never quite justified the cost. So after letting the orphan sit idle for several years, the MP completely rebuilt it.

Conversion first as a poppet-valves testbed, then with roller bearings, removal of the third cylinder, oil firing, and Elesco feedwater heater. When completed, the new/old engine had two 27 x 28" cylinders, a boiler pressed to 200 psi and possessing 4,028 sq ft of heating surface, thermic syphons in the firebox..

Although tractive effort remained about the same, the overhaul transformed the now-renumbered 6001 and she proved a powerful and reliable engine until her 1952 retirement. Her only weak spot was the "poor man's feedwater heater," the Elesco ES. This proved a problematical device that was very sensitive to bad water. When on her mettle, however, the 6001 hit 90-95 mph regularly.

Even so, the MP saw no particular need to retrofit any of its other Pacifics in a similar way.

See WM Adams (Trains, November 1978) for a full account of the modified 6001 "Madame Queen" and the prodigies of speed of which she was capable.

Class P-69 (Locobase 151)

Data from "New Six-Coupled Passenger Locomotive", American Engineer and Railroad Journal, Volume 67, No 8 (August 1902), pp. 236-237. See also "Passenger Engines for the Chesapeake & Ohio and the Missouri Pacific", Railroad Gazette, Volume XXXIV [34], No 31 (1 August 1902), pp.603-605; and MP 1924 Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. (Many thanks to Chris Hohl for his 22 September 2017 email reporting unlikely boiler pressure values for 177 entries. A Locobase macro caused the error .) Works numbers for the MP engines were 26614-26618 in June 1902, 28312-28314 in August 1903, and 28410 in April 1904. St Louis, Iron Mountain & Southern locomotives were delivered simultaneously, works numbers 26619-26623 in July 1902 and 28315-28321 in August 1903.

Two number sequences, the first for Missouri Pacific, the other for Iron Mountain. The 1905 renumbering put in the 6501-6521 sequence.

Among the first Pacifics, these had the then-common inside valve motion and trailing-truck bearings. Piston valves measured 11" (279 mm) in diameter. Firebox heating surface included 22 sq ft (2.04 sq m) of arch tubes.

All were rebuilt with Baker valve gear (except one with Walschaerts) driving outside piston valves and the steam was superheated; see Locobase 9396. Tender capacity increased to 7,000 US gallons (26,495 litres) of water and 14 tons (12.7 metric tons) of coal.

The last left service in 1953.

Class P-69 - superheated (Locobase 9396)

Data from MP 1924 Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange.

Locobase 151 shows these pioneering Pacifics in their original saturated-steam configuration. All were rebuilt with Baker valve gear (except one with Walschaerts) driving outside 11" (279 mm) piston valves and the steam was superheated. In the process, the cylinder diameter grew by an inch and, as usually happened in these early conversions, the boiler lost some of its total evaporative heating surface. Later retrofits added 10 sq ft (0.93 sq m) of arch tubes and 36 sq ft (3.34 sq m) of thermic syphons.

The last left service in 1953.

Class P-73 (Locobase 14957)

Data from "New Locomotives for the Missouri Pacific", Railway Mechanical Engineer, Vol 95, No 10 (October 1921), pp. 609-613; and F G Lister, "Locomotive Boiler Efficiencey by Proper Circulation of Water", The Boiler Maker, Vol 22, No 9 (September 1922), pp. 256-257. (My thanks to Chris Hohl for his 21 February 2013 inquiry that led to several new entries and updates of several more.) Built at Schenectady in April 1921.

Like the other locomotives ordered by the MoPac from Alco in this year, the Pacifics were fitted with Harter Circulating Plates. Locobase 14956 has a full description of this device. (Briefly, a solid plate laid horizontally among the superheater flues and pierced for 2" diameter steam risers sought to encourage circulation from bottom to top.) Arch tubes added 26 sq ft (2.4 sq m) to direct heating surface area and 14" (356 mm) piston valves supplied steam to the cylinders.

These were later rebuilt by the MP with a new boiler; see Locobase 14958.

Class P-73 (Locobase 14958)

Data from MP 7 - 1956 Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. (My thanks to Chris Hohl for his 21 February 2013 inquiry that led to several new entries and updates of several more.)

The quintet of express passenger Pacifics bought by the MP in 1921 from Schenectady (Locobase 14957) were continuations of a larger class originally built in 1910-1912. This later batch was fitted with Harter circulating plates (see the earlier entry for full description and bibliographic references).

At a later time, the MP reworked the boiler and firebox considerably. Locobase can't be sure, but he believes that the Harter plates were removed, but the tube count increased by 16. Two thermic syphons replaced the arch tubes in the firebox and contributed 65 sq ft (6 sq m) to direct heating surface. They were fitted with Timken roller bearings, a BK stoker, and power reverse gear.

Class P-73 / 6601 (Locobase 9397)

Data from MP 7 - 1956 locomotive diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. Schenectady works numbers were 65203-65212 in March 1924 and 66017-66025 in December. Brooks completed the order with works numbers 66264-66273 in April 1925.

Firebox had combustion chamber. Drury (1993) notes that the first 10 were delivered as 6445-6454; they were renumbered in 1934. The design was an interesting combination of the USRA's Heavy Pacific boiler (Locobase 173) and the Light Pacific's 73" drivers (Locobase 172). Cast trailing trucks were standard. 6614-6619 were oil-burners to provide clean power for the MP's Sunshine Special.

As often happens in such classes, modifications over time ensured variety. By the 1940s, Drury tells us, "...no two engines in the class had the same combination of disk main drivers, roller bearings, pilots, and tender."

The clas was retired in 1949-1954.

Class P-73/1151 (Locobase 153)

Data from tables in 1930 Locomotive Cyclopedia, supplemented by MP 8 - 1936 locomotive diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange.

Firebox had combustion chamber. Ran in Texas for the International-Great Northern (works numbers were 66699-66703 in November 1926; road numbers were 1151-1155) and St. Louis, Brownsville & Mexico (works numbers were 67305-67310 in June 1927, road numbers1156-1161). The IGNs had Baker gear, Worthington SA feedwater heaters, the others Worthington BLs.

Class P-73/6401 (Locobase 9395)

Data from MP 1924 Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. (My thanks to Chris Hohl for his 21 February 2013 inquiry that led to several new entries and updates of several more.) Works numbers 48319-48338 in October.

The first half of a large batch of 73" driver Pacifics were delivered with saturated boilers of considerable size (in the top 10% of all 4-6-2s produced at the time). Their fireboxes and grates were on the small side as was the locomotive's cylinder volume. 12" (305 mm) piston valves supplied steam to the cylinders.

The design wasn't allowed to remain that unbalanced for long. See Locobase 1373 for the much-altered superheated upgrade, which featured larger cylinders fed by larger piston valves.

Class P-73/6401 rebuilds (Locobase 16441)

Data from MP 1956 Locomotive Diagrams- Steam supplied in August 2013 by Allen Stanley from his extensive Rail Data Exchange.

As noted in Locobase 9395, 20 Pacifics were delivered as saturated-steam engines rolling 69" drivers. From the first days, it must have been apparent that the boiler-cylinder volume balance was way off. So before very long, the MP updated the original design with superheaters, larger cylinders with longer strokes and piston valve diameters increased to 14" (356 mm).

In later years, the firebox was modified with 65 sq ft (6.04 sq m) of thermic syphons, which increased the total firebox heating surface to 271 sq ft (25.18 sq m). The class also received 73" (1,854 mm) drivers and some had Timken roller bearings and Elesco feedwater heaters. Sources differ on whether this class had 26" cylinders after the modifications, but it is certain that by the end of their careers, all of the 6401-6420 class used 25" x 26" cylinders.

In the early 1950s, the final count of oil-burners with 10,000 gallon (37,850 litres) tenders stood at five; the rest were coal burners that used BK stokers.

Retirements began in 1948 with 6410, but most served into the early 1950s. 6414 was the last to be dismissed in August 1954.

Class P-73/6420 (Locobase 1373)

Data from MP 1924 Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. (My thanks to Chris Hohl for his 21 February 2013 inquiry that led to several new entries and updates of several more.) Works numbers were 50573-50582 and 50605-50608 in January 1912, 53710-53714 in July 1913.

The 6420-6439 were follow-ons to the 6401 engines (Locobase 9395), but were much bigger. The cylinder volume grew considerably with the adoption of square cylinders (bore equalled stroke) ; cylinder volume grew by a third. In the process, , piston valve diameters increased to 14" (356 mm) and total evaporative heating surface actually increased even with the addition of superheaters. 1921 saw the delivery of five more (6440-6444) fitted with Harter circulating plates (Locobase 14958).

In later years, the firebox was modified with 65 sq ft (6.04 sq m) of thermic syphons, which increased the total firebox heating surface to 271 sq ft (25.18 sq m). The class also received 73" (1,854 mm) drivers and some had roller bearings and Elesco feedwater heaters.