2-8-8-4

The 2-8-8-4 "Yellowstone" Locomotive

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. 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. 72 Yellowstone type locomotives were built for four different American railroads, and four narrow gauge locomotives for a Brazilian railroad.

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 at 63-64". For greater speeds, the Union Pacific Railroad chose a 4-wheel leading truck and 68" drivers for its Big Boy 4-8-8-4 class.

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

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 in area. The purpose of this was to burn Rosebud coal, a very low-quality lignite 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. Built to burn coal, they were converted to oil firing.

Duluth, Missabe & Iron Range

The Duluth, Missabe & Iron Range is 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. Prior to the advent of the diesel locomotive, the eighteen Yellowstones built by Baldwin for the road were the ultimate power available. They were based on the Western Pacific's successful 2-8-8-2 locomotives, but the longer cab and other equipment ordered by the DM&IR necessitated a four-wheel trailing truck.

Eight locomotives (class M-3) were built in 1941, and ten more (class M-4) were delivered in 1943. At that time the DM&IR was experiencing a downturn in traffic, and a number of the locomotives were leased to and delivered directly to the Denver & Rio Grande Western, in whose service they excelled for several years before finally being delivered to their owners.

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 stack, while others used a Worthington unit with its rectangular box in the same location.

They were the most powerful Yellowstones, producing 140,000 lbs of tractive effort, and had the most weight on drivers so that they were no more prone to slipping.

Three of the eighteen built still survive.

Baltimore & Ohio

The Baltimore & Ohio Railroad took delivery of 30 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, possibly the only 2-8-8-4s outside the United States, and had the largest boilers ever used on a narrow-gauge Mallet.

This text has also been posted on Wikipedia and is entirely the work of the author (myself), who retains all copyright.