The Pennsylvania Railroad's 52 T1 class duplex-drive 4-4-4-4 steam locomotives, introduced in 1942 (2 prototypes) and 1945-1946 (50 production), were the last steam locomotives built for the PRR and arguably its most controversial. They were ambitious, technologically sophisticated, powerful, fast, and distinctively streamlined by Raymond Loewy. However, they were also prone to wheel slip both when starting and at speed, complicated to maintain, and expensive to run. The PRR decided in 1948 to place diesel locomotives on all express passenger trains, leaving unanswered questions as to whether the T1's flaws were solvable, especially taking into account that the two prototypes did not have the problems inherent to the production unit. An article appearing in a 2008 issue of the Pennsylvania Railroad Technical and Historical Society Magazine showed that inadequate training for engineers transitioning to the T1 may have led to excessive throttle applications, resulting in driver slippage. Another root cause of wheelslip was faulty "spring equalization": The stiffnesses of the springs supporting the locomotive over the axles were not adjusted to properly equalize the wheel-to-track forces. The drivers were equalized together but not equalized with the engine truck. In the production fleet, the PRR equalized the engine truck with the front engine and the trailing truck with the rear engine, which helped to solve the wheel slip problem.
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