PRR T1
with the Manhattan Limited to New York.
The Pennsylvania Railroad's 52 T1 class duplex-drive 4-4-4-4 steam locomotives, introduced in 1942 (2 prototypes) and 1946 (50 production) were their last-built steam locomotives, and their most controversial. They were ambitious, technologically sophisticated, powerful, fast, and uniquely streamlined by Raymond Loewy. Against that, they were prone to violent wheelslip, complicated to maintain, and expensive to run. In 1948, the PRR vowed to place diesel locomotives on all express passenger trains, leaving unanswered whether the T1's flaws were soluble.
Development The last production express passenger class the PRR had produced was the K4s of 1914, produced until 1928. Two experimental enlarged K5s locomotives were produced in 1929, but they were not considered enough of an improvement to be worthwhile. After that point, the PRR's attention switched to electrification and the production of electric locomotives; displaced steam locomotives meant that the railroad had an excess of steam power and no real need for additional locomotives. However, the deficiencies of the K4s became more and more glaringly obvious as the 1930s progressed. They were fine locomotives, but as train lengths increased, they were simply not big enough for the task. Double headed K4s locomotives became the norm on heavy trains. The railroad had the locomotives to spare, but paying two crews and running two locomotives per train was simply not the most economical choice. Meanwhile, other railroads were leaping ahead, developing larger and larger passenger power. Rival New York Central built better and better Hudsons, while other roads developed passenger 4-8-2 "Mountain" types and then 4-8-4 "Northern" designs. The PRR's steam power began to look rather outdated indeed. In the mid to late 1930s, the PRR began to develop steam locomotives again, but with a difference. Previous PRR locomotive policy had been unrelentingly conservative, but new, radical designs took hold. Designers from the Baldwin Locomotive Works, the PRR's longtime development partner, persuaded the railroad to adopt Baldwin's latest revolutionary idea; the duplex locomotive. This split the locomotive's driving wheels into two sets and gave each set its own pair of cylinders and rods. Previously, the only locomotives with split sets of drivers were articulated locomotives, but the duplex used one rigid frame. In a duplex design, cylinders could be smaller, pistons could be slower, and the weight of side and main rods could be drastically reduced. Given that the movement of the main rod cannot be fully counterbalanced, the duplex design would drastically reduce "hammer blow" on the track. The first PRR duplex was the single experimental S1 of 1939. This proved successful, but it was simply too large a locomotive, and its size prohibited it from operating over most of the PRR's network. The concept being seemingly proven, the PRR returned to Baldwin, to develop a duplex design fit for series production.
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