The name Decapod was commonly applied to steam locomotives with ten driving wheels, in other words five driven axles. Most commonly, it was employed for locomotives of the 2-10-0 classification in the Whyte notation, although it was early on also applied to 0-10-0 locomotives, more commonly known as ten-coupled.
The term Decapod for such locomotives is mostly an American usage, and thus in this node I am covering purely the American usage of this type. European and others will be covered under 2-10-0.
The first Decapods were built for the Lehigh Valley in 1867; proving too rough on the track thanks to their long coupled wheelbase, one pair of drivers were removed. No more followed for 24 years, until the Erie bought six for pusher service between 1891 and 1893. In that low-speed service where high tractive effort was the most critical attribute, these Decapods were successful. Small numbers of other Decapods were built over the next twenty years, mostly for service in steeply graded mountainous areas where power at low speeds was the requirement. The type did not prove that popular, however, compared to the wildly successful Consolidation (2-8-0) type. Among the Decapods placed in service were a number for the Santa Fe, of interest mostly because they were tandem compounds.
The first great boost in the number of Decapods in service was thanks to historical events. Imperial Russia ordered approximately 1200 Decapods from American builders during World War I. When the Bolshevik revolution took place, over 800 had already been delivered, but more than 200 were either awaiting shipment or were in the process of construction. These stranded locomotives were adopted by the United States Railroad Administration, the body created by the Government to oversee and control the railroads during the War, converted to American standards, and put to use on American railroads. Small and light-footed, these Russian decapods proved popular with smaller railroads, and many of them remained in control long after the USRA's control of the railroads ceased. Many indeed lasted until the end of steam on those railroads.
The Pennsylvania Railroad, however, soon became the biggest user of the Decapod in the United States. The type was ideally suited to the Pennsy's heavy graded Allegheny Mountains routes; power and lugging ability, not speed, was what was called for. The PRR bought 598 of the brutes, building 123 itself and then ordering the rest from Baldwin in one of the biggest locomotive orders of all time. The PRR decapod, class I1s, was not a dainty, light-footed beast like the Russian decapod; it was huge, taking advantage of the PRR's heavy trackage and high allowed axle loading, with a fat, free-steaming boiler that earned the type the nickname of 'Hippos' on the PRR. Giant cylinders enabled the I-1s to put down that power to the rails, and giant tenders allowed such hungry and thirsty beasts to work hard and long between stops. Their power and sheer brute force was undeniable, but they were not popular with the crews, for they were hard riding at all but low speeds.
Following that, a small number of other Decapods were ordered by other railroads; those built for the Western Maryland were the largest ever built, at almost 420,000 lbs (190,500 kg) weight.
The Decapod's main advantage was that five out of six of its axles were powered ones, meaning almost all the weight was available for traction rather than being wasted on guiding axles. This long coupled wheelbase, however, caused problems on tightly curved track, so blind drivers were the norm on the central axle and sometimes others, often coupled with lateral motion devices on the leading driven axles.
Its disadvantages included the fact that the firebox size was restricted by having driving wheels underneath it. Either it had to be fitted in between the wheels (common on earlier locomotives) and was then suboptimally long and narrow, or it could be mounted above the wheels, making it sufficiently wide and long but shallower than was optimal. Most later locomotives chose the latter route. The firebox being so mounted also restricted the diameter of the driving wheels, making them too small to carry sufficient counterweights to counterbalance the weight of the heavy main rods and side rods required to absorb the powerful thrust of the cylinders on the larger locomotives. Thus, they were not very well balanced at speed and rode very roughly; they were generally not permitted speeds of greater than 50mph (80 kph).
Thirteen Decapod locomotives survive in the USA, including six Russian Decapods and one PRR I-1s. Two, including one Russian decapod, are operational.
Thanks to: An article in Trains magazine; the Model Railroader Locomotive Cyclopedia Volume I, Steam Locomotives; Pennsy Power by Alvin F. Staufer; and steamlocomotive.com for its list of surviving North American steam locomotives.