Under the Whyte notation for the classification of steam locomotives, a 2-8-8-4 is a locomotive with two leading wheels, two sets of eight driving wheels, and a four-wheel trailing truck.

Other equivalent classifications are:
UIC classification: 1DD2 (also known as German classification and Italian classification)
French classification: 140+042
Turkish classification: 45+46
Swiss classification: 4/5+4/6

The equivalent UIC classification is, refined for Mallet locomotives, (1'D)D2'.

Such a long locomotive must be an articulated locomotive, and all the examples produced were of the Mallet type, having a hinged joint between the first and second groups of driving wheels, and having the superstructure of the locomotive rigidly attached to the rearmost set, with the foremost set and leading truck allowed to swing sideways on curves.

The type was generally named the Yellowstone, a name given it by the first owner, the Northern Pacific Railway, whose lines run near Yellowstone National Park. Seventy-two Yellowstone type locomotives were built for four different American railroads.

The 2-8-8-4 turned out to be the common choice of arrangement for the very largest steam locomotives when the speeds required were only moderate. All classes of Yellowstone had fairly small drivers of 63 to 64 inches (1.60 to 1.63 m). For greater speeds, the Union Pacific Railroad chose a 4-wheel leading truck and 68 inch (1.73 m) drivers for its Big Boy 4-8-8-4 class.

Several classes of Yellowstone, especially the Duluth, Missabe and Iron Range's locomotives, are among the largest steam locomotives of all time, the exact placing being dependent on what criteria are being used.

Northern Pacific

The Northern Pacific Railway was the first railroad to order a 2-8-8-4. The first was built in 1928 by ALCO; at the time, it was the largest locomotive ever built, and it was equipped with the largest firebox ever applied to a steam locomotive, some 182 square feet (17 m²) in area. The purpose of this was to burn Rosebud coal, a very low-quality coal the NP could obtain very cheaply on-line. Unfortunately, that firebed was just too large for the available draft, and the fire burned poorly. The problem was mitigated by blocking off the first few feet of the grates. Baldwin built eleven more for the NP in 1930.

Southern Pacific

The Southern Pacific Railroad's famous "cab-forward" articulated steam locomotives were effectively a Yellowstone in reverse, but the SP also owned some conventional 2-8-8-4s for use on its less mountainous routes. Twelve AC-9 class locomotives were built by Lima in 1939; they were attractive machines, with skyline casings and striped pilots. Originally built to burn coal, they were later converted to oil firing.

Duluth, Missabe and Iron Range

The Duluth, Missabe and Iron Range Railway was an iron ore hauling railroad in Minnesota. Iron ore is a heavy commodity and the DM&IR operated long trains of ore cars, requiring as much power as the railroad could get their hands on. The design of these locomotives was based upon a series of 10 powerful 2-8-8-2s that Baldwin had built previously for the Western Pacific Railroad. The need for a larger, coal burning firebox and a longer, all-weather cab led to the use of a 4 wheel trailing truck, giving them the "Yellowstone" wheel arrangement. They were the most powerful Yellowstones built, producing 140,000 lbf (623 kN) of tractive effort, and had the most weight on drivers so that they were not prone to slipping as were other Yellowstones.

Eight locomotives (class M-3) were built by Baldwin in 1941. The Yellowstones met or exceeded the DM&IR specifications so ten more were ordered (class M-4). The second batch was completed late in 1943 after the Missabe's seasonal downturn in ore traffic, so some of the new M-4s were leased to and delivered directly to the Denver & Rio Grande Western.

The next winter, the D&RGW again leased the DM&IR's Yellowstones as helpers over Tennessee Pass, Colorado and for other mainline freight duties. The Rio Grande returned the Yellowstones after air-brake failure caused Number 224 to wreck on the Fireclay Loop. This was despite the Rio Grande's earlier assessment that these Yellowstones were the finest engines to ever operate on the railroad.

DM&IR's locomotives were the only Yellowstones equipped with a high-capacity pedestal or centipede tender, and had roller bearings on all axles. Some of the locomotives were fitted with the cylindrical Elesco feedwater heater before the smoke stack, while others used a Worthington unit with its rectangular box in the same location.

Only one Yellowstone was retired before dieselization took place on the Missabe; Number 237 was sold for scrap after it was involved in a wreck. The rest of the 2-8-8-4s were retired between 1958 and 1963 as diesel locomotives completely took over on the Missabe Road.

Three of the eighteen built still survive and are on display Number 227 in Duluth, Minnesota, Number 225 in Proctor, Minnesota, and Number 229 in Two Harbors, Minnesota.

Baltimore and Ohio

The Baltimore and Ohio Railroad took delivery of 30 Class EM-1 Yellowstones in 1944 and 1945, the smallest and last Yellowstones built and the largest group. They were the largest locomotives on the B&O and the last articulated locomotives delivered to them. War production restrictions gave B&O steam locomotives when they would have preferred diesels, but they performed well. All were out of service by 1960 as dieselization swept them away.

Outside the United States

The metre gauge (narrow gauge) Central Railway of Brazil took delivery of four 2-8-8-4s from the German firm of Henschel in 1937. They were the only narrow gauge locomotives of this wheel arrangement, but not the only 2-8-8-4s outside the United States, they had the largest boilers ever used on a narrow-gauge Mallet.

Soviet Russia constructed two 2-8-8-4 locomotives at the Kolomna works. These were the P38 Class numbers P38.001 and P38.002. The first locomotive carried partial casings over the boiler and smokebox typical of the 1950s. P38.002 bore no such adornments and had a more conventional look. Both engines had tenders with part bogie and part fixed frame similar to the American 'centipede' tenders.

References

• Barris, W. The Yellowstone Type Locomotive. Retrieved Jan 4, 2003 from http://www.steamlocomotive.com/yellowstone/
• Westcott, L. (Ed.). (1960). Model Railroader Cyclopedia Volume 1: Steam Locomotives. Waukesha, WI: Kalmbach. ISBN 0-89024-001-9
• Rakov. V.A. Locomotives of the Russian and Soviet Railways - Vol 1 - 1845 to 1955 Moscow-1995

External links

• 2-8-8-4 or Yellowstone
• The Yellowstone Type Locomotive
• Web Site of ToyTrains1 2-8-8-4 Yellowstone Steam Locomotives