Northern Pacific 2-8-8-4 "Yellowstone" Locomotives in the USA

Class Z-5 (Locobase 335)

Data from tables in 1930 Locomotive Cyclopedia, confirmed by locomotive diagrams from 1949 hosted on [] (7 Feb 2004). See also DeGolyer, Volume 82, pp. 307+; and "Largest Locomotive Ever Built - Northern Pacific Railway", Railway Journal, Volume 35, No 2 (February 1929), pp. 22; and NP Locomotive Data Cards supplied in August 2013 by Allen Stanley from his extensive Rail Data Exchange collection. (Thanks to Bill Hoehn, whose 11 February 2013 inquiry about the 1941 update led to Locobase's revision of this entry and to Chris Hohl, whose 7 March 2015 email provided a correction on the tender's coal capacity; and Ray Uhlich, whose 10 November 2020 email supplied the reason for awarding the production contract to Baldwin.) Baldwin works numbers were 61292-61293 in April 1930; 61356-61358, 61380-61381 in May; 61419-61420, 61434 in July; and 61495 in September.

These were known as "Yellowstones" and gave the name to the wheel arrangement. The first engine (5000) was built by Alco-Schenectady and had works number 67578. Its adhesion weight came to 553,000 lb (2251,230 kg) and total engine weight amounted to 717,000 lb (325,226 kg). Maximum axle loading was 70,500 lb (31,978 kg).

Locobase originally began this paragraph with "Then for some reason, this mostly Alco road then gave the production order to Baldwin." Ray Uhlich gave the simple answer in his email. According to his father, a mechanical inspector for the NP, Baldwin bid a lower amount for the contract. Locobase adds that especially considering the country had begun its descent into the Great Depression, building large articulateds must have extended quite a few jobs.

Note the large firebox suitable for burning low-calorie "Rosebud coal." Valve motion limited cutoff of the 14" (356 mm) piston valves, which had a maximum travel of 7 1/2" (191 mm). When the original firebox proved too large to sustain a high-enough temperature and permit complete combustion of the fuel, the fix was installing a Gaines' wall two feet from the front of the firebox, effectively reducing the size of the grate, but increasing the interior temperature. The brick arch was to be located such that the area above the arch was at least 100% of the flue area. (For a full description of the Gaines combustion chamber's intended effect on combustion, see Locobase 4228.)

The Q&C "Nogroth" grate seems to have been a rosebud rocker design, intended "to burn sub-bituminous coal.". The 3/8" (9.5 mm) diameter hole provided air and the perforated plates rocked "both ways" with a 4 1/2" (114.5 mm) opening.

Three Nicholson thermic syphons in the firebox and two in the combustion chamber contributed 256 sq ft (23.8 sq m) to direct heating surface area. The combustion chamber itself added another 162 sq ft (15.05 sq m). The design also featured Type B Dupont Simplex stokers with a capacity of 20 short tons (18,144 kg) of coal per hour, Alco Type G power reverse gear, and a Coffin feed in water heater.

A 1941 update installed Timken roller bearings in December 1941 at the cost of $19,059.74 for each engine.

Tonnage rating over a ruling grade of 1.1% was 4,000 tons (3,636 metric tons). This class was given a sturdier front frame, had steam leaks sealed, and run at a higher boiler pressure, producing 145,930 lb (66,193 kg) of tractive effort. The NP removed the feed water heater and boosters in 1948-1949.