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 as Raymond Loewy's streamlining caused smoke to flow low over the engine, often obstructing the enginemen's view.

The T1s handled the PRR's premier passenger trains in non-electrified territory in the mid-1940s. They were known for their speed, and had the horsepower to pull a 16-car passenger train at a steady 100 miles per hour — outperforming the 5400-horsepower diesels of the day, and making it possible to replace double-headed K4s Pacifics with a single locomotive. They were 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 at speed. (However, some authorities 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 Walschaerts valve gear, while another 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. None of the T1s survive — the last few went to scrap in 1954, according to one source — but they will be remembered as a bold attempt to meet the challenge of the diesel on its own terms.