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The K28, K2, and K3 were the Pennsylvania's first examples of a 4-6-2 Pacific passenger steam locomotive. The first of this type ordered by the PRR was the lone K28, bought from the Pittsburgh Works of the American Locomotive Company (ALCo). It was given the road number 7067 and had 80" drivers, 24" x 26" cylinders, a boiler pressure of 205 psi, exerted 32,620 lbs of tractive effort and weighed 273,600 pounds. After being put through the rugged and strict testing the PRR ws famous for, the company decided that this locomotive type was what it needed for passenger service. Development began on more "personalized" engines better suited to the unforgiving mountain terrain which made up the majority of the PRR's mainline.
K2 and K3 classesEdit
These were near copies of the original ALCO locomotive, sharing many characteristics. The first K2, built by the PRR's Altoona Locomotive works in 1910, had duplicate driver diameter, cylinder diameter, tractive effort, and boiler pressure, though they were slightly heavier at 278,00 pounds. From 1910 through 1911, the Altoona Locomotive Works manufactured 153 examples of this class. In late 1911, the K2 design was modified to handle automatic stokers via elevation of the firing deck. The Juniata Shops (adjacent to the ALW) modified 62 locomotives from 1911 through 1913 to be compatible with the stoker. These were designated K2a. PRR also placed an order for 10 K2a's with ALCo, who delivered them with raised firing decks in 1912. Although these 72 locomotives were modified to handle auto-stokers, only a small number were actually equipped with them. A pair of new Pacifics were built by ALW in 1911, which were identical to the K2 class except for their 72" drivers. These were given the designation K2b, with road numbers 3371 and 3375 assigned. In 1913, Baldwin Locomotive Works built 30 new Pacifics similar to the K2 class, but with 26" diameter cylinders, superheater equipment and mechanical stokers. These were designated the K3s class. A total of 257 locomotives were built based off the original K28.
By 1913, PRR had placed another order with ALCO for a further improved type of Pacific, which it classed as the K29. This engine featured multiple improvements over the previous models, including a very large boiler and brick arch firebox. It also incorporated outside steam delivery pipes, stoker, screw reverser, and superheater. The specifications consisted of 80" drivers, 27" x 28" cylinders, a 200 psi boiler pressure, a tractive effort of 43,375 lbs, 197,800 lbs on the drivers and a total weight of 317,000 pounds. This engine would form the basis for a copy built by PRR at its Altoona Locomotive Works in 1914. It was classed K4s and given road number 1737. The railroad considered it successful, however, because of a high-priority order for 2-8-2 Mikados, the ALW was too busy to mass produce the new locomotive until 1917. Once production could finally being, the K4s was built through 1928, with the Juniata Shops building a total of 349 of the K-4s Pacifics, while the Baldwin Locomotive Works built 75 of them. Largely, the K4 class received very little major modifications. However, experiments abounded in streamlining, valve gears, and the modern slat pilot with retractable coupler. Streamlining experiments spanned the early 1930's-late 40's and consisted of various shrouds from such famous industrial designers as Raymond Leowy and Richard Dreyfuss. Engine #3768 was streamlined with a shroud by Leowy. This design almost completely enveloped the locomotive's boiler an running gear in a cylindrical shape, thus earning the nickname "Topedo" from her crews. As with other such designs, this made routine maintenance difficult and the design was not copied. Four more engines, 1120, 2665, 3678, and 5338, were concealed in a less overall streamlining designed with maintenance in mind. It left most of the running gear accessible to workers. This practice was at least inspired by the NYC's Dreyfuss-designed streamlined 4-6-4 Hudsons. K4s #1188 was only streamlined with a "skyline" casting which smoothed the top of the locomotive's domes and fittings in a manner similar to that of he Southern Pacific's G-series 4-8-4's. These various designs were tested for efficiency by the PRR, and it was soon decided that the castings did not contribute enough to speed or efficiency to be worth the effort needed to maintain them, and thus a large fleet of streamliners was never built. Other modifications to K4s locomotives include smoke deflectors, counter-balanced coupling-rods, and roller bearings. Despite having over 420 locomotives built, only 2 have not been scrapped. Engine #3750 was given to the Railroad Museum of Pennsylvania in Strasburg, PA, while #1361 was donated to the city of Altoona itself, which placed it on display at the Horseshoe Curve until the foundation of the Railroader's Memorial Museum, which promptly removed it from display and had Conrail restore it to operational status in 1985. The locomotive pulled excursions until 1987, when a minor derailment caused an axle failure. Since then, the Museum has been unable to re-restore it to operational status as a result of poor money handling.
In 1929, the PRR performed tests and research to see if a larger Pacific then the K4s would be economical. These tests culminated in the construction of two prototype locomotives, classed K5. They were both fitted with a much wider boiler than the K4s, but one dimensionally similar to those of the I1s class 2-10-0 decapods. Most other dimensions were enlarged over the K4s as well; except the 70 square feet (6.5 m2) grate area and the 80 in (2.0 m) drivers. In short, the tests of the two prototypes resulted in the conclusion that it would be unnecessary to manufacture a replacement for the K4s series which was still a Pacific. Instead, the PRR used a radically different Duplex-drive design.