The 2-6-6-4 was another example in a growing trend of larger and more powerful steam locomotive designs. At the time of its development many of the late era, highly advanced rigid and articulated models were either already in service or soon to be built. For instance, these wheel arrangements included the 2-8-4 Berkshire, 4-8-4 Northern, 2-10-4 Texas, 2-6-6-2 Mallet Mogul, 2-8-8-2 Chesapeake, 2-8-8-4 Yellowstone, 4-6-6-4 Challenger, and Southern Pacific's unique Cab Forwards. By the 1930s railroads were looking for locomotives which could not only offer greater horsepower in moving increasingly heavier freight (and passenger) trains but also do so at higher speeds. One way in which this was accomplished was to increase the size of the firebox, first developed by the Lima Locomotive Works, and this concept led to the line of superpower steam designs mentioned above.
The first 2-6-6-4s put into service were built by the Baldwin Locomotive Works in 1934 for the Pittsburgh & West Virginia Railroad, a small Class I system that ran from Connellsville, Pennsylvania, through Pittsburgh and connecting to Pittsburgh Junction, Ohio. The P&WV listed their fleet as Class P-1 and P-2, numbered 1100-1106, and were by far the largest steamers the road ever put into service. They offered a 250 boiler psi, a Belpaire firebox with a grate area of 102 feet, weighed more than 450 tons, and offered a tractive effort of more than 100,000 pounds. Interestingly, however, the railroad did not use theirs in high-speed freight service but instead they were normally assigned in slow drag service moving heavy trains over the P&WV's stiff grades. Still, the success of the design proved to Baldwin that other roads would likely also find interest in the locomotive.
Curiously, of all the lines which may been interested in the 2-6-6-4 it was a southern system which purchased the wheel arrangement next. A year after the P&WV acquired its first example the Seaboard Air Line went to Baldwin and ordered a small roster of five in 1935. Numbered 2500-2504 and listed as Class R-1 the locomotives were simple expansion designs featuring Baker valve gear, tractive efforts above 82,000 pounds, and rated at 2,700 tons. Two years later Seaboard went back to Baldwin for a secondary order, Class R-2 #2505-2509 that utilized Walschaert valve gear but for the most part were very similar to the R-1s. According to the article, "Twin Stacks, 'Ready Boys? I'm Going To Wind Her Up'" from the August 1979 issue of Trains the 2-6-6-4s were the largest steamers ever used by the Seaboard, whose earlier 2-8-8-2s were not very well liked and sold to the Baltimore & Ohio.
Along with the Clinchfield the Seaboard was the only southern road to use articulated steam in the Deep South. While the SAL's R-1s and R-2s might be considered less powerful than other versions of the design (such as the P&WV's or N&W's) they are often regarded as the first articulateds built for high-speed operation sporting 69-inch drivers. Overall, the locomotives were said to be well-liked by SAL crews who were somewhat disappointed in having to operate the big steamers at slower speeds than they were capable of running. After just a decade of service the Seaboard sold their fleet to the B&O, which reclassified them as KB-1/a and used them in fast-freight service along the eastern end of the Cumberland Division between Cumberland and Brunswick, Maryland where grades were relatively flat in the Potomac Valley.
The Norfolk & Western, of course, made the 2-6-6-4s famous when it
began outshopping its Class As in 1936 built in Roanoke, Virginia. In
all the N&W would roster forty-three examples, numbered 1200-1242,
that were clearly the finest ever built. Normally, the railroad used
them along its Pocahontas Division in West Virginia as well as running
through Portsmouth, Ohio and Columbus. According to SteamLocomotive.com
the Class As were rated at 13,000 tons in slow drag service (with a
tractive effort surpassing 125,000 pounds) but could also pull a
passenger consist at an incredible 70 mph! The locomotives remained in
service until steam finally ended on the N&W in 1959. Finally, of
note was the 2-6-6-4 designed by Baldwin but never built for the Erie.
Had it taken place, the railroad would have been operating the big
Mallet five years prior to anyone else.
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