The Pennsylvania Railroad's T1 4-4-4-4 duplex drive locomotive has become a legend, for due to the advance of dieselization it had a brief service life. The first two (Nos. 6110-6111) were designed and built by Baldwin Locomotive Works in 1942, and the remaining 50 were erected in 1946 by the Pennsylvania's own Altoona Shops (Nos. 5500-5524) or by Baldwin (Nos. 5525-5549). The striking "shark nose" styling was by famed designer Raymond Loewy, and was emulated not only by Baldwin's "shark nose" diesels but by a group of 4-8-4s built for the South Australian Railways. This Baldwin builder's photo of Pennsylvania Railroad 4-4-4-4 No. 5526 shows the T1 class as originally built, with the "portholes" reminiscent of Buicks of the 1940s and 1950s. This image from the collection of my brother, David V. Leonard, shows off the original sheen of the T1's Brunswick green finish. In operation their paint job quickly faded to dingy black because Loewy's streamlining caused smoke to flow low over the engine, often obstructing the enginemen's view.

The T1 was equipped with oscillating-cam poppet valves for more precise timing of steam admission to, and exhaust from, the cylinders. Their dual drive, plus the absence of valve gear cranking on the drivers, meant their reciprocating parts were lighter than those of a conventional 4-8-4. The dual drive also made possible a shorter piston stroke for the same power output. At the same time, the T1s were placed in regular service before all their "bugs" had been worked out. They were known for hard-to-control driving wheel slippage, not only while starting a train but also at speed. (Some authorities, however, claim this was due to poor throttle handling by inexperienced engineers.) The PRR tried to fix the slippage problem by outfitting one engine, No. 5547 reclassified T1a, with conventional Walschaert valve gear, while No. 5500 was given rotary cam poppet valves. But the engines began to be placed in storage around 1948, and thus were not in service long enough to correct the defects. For their specifications, see the commentary on No. 5527.