PRR T1 - Development

Development

The last production express passenger class the PRR had produced was the K4s of 1914, produced until 1928. Two experimental enlarged K5 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 The PRR's steam power began to look rather outdated indeed.

The PRR began to develop steam locomotives again in the mid-to-late 1930s, 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 faster, 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 lower the weight of the reciprocating mass meant that higher speeds could be more easily achieved. Use of poppet valves also increased the speed each set of drivers could achieve because they gave very accurate steam delivery to the cylinders. However, there was a drawback of the metallurgy used, meaning that the poppet valve could not take the stress of sustained high speed operation (meaning over 100 mph (160 km/h). on production T1s).

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. The two Baldwin prototypes (#6110 and #6111) delivered glowing test reports, resulting in a production order for 50 T1s, split between the PRR's own Altoona Works and Baldwin. The last production T1 (#5549) entered service on August 27, 1946.

Engine #5539 developed 5,012 hp (3,737 kW), as tested between September 11, 1946 and September 14, 1946 by C&O dynamometer car DM-1 while on loan to the C&O. In 1944 #6110 tested in Altoona, developed 6,550 hp (4,880 kW) at 85 mph (137 km/h).

Due to their high level of complexity relative to other steam locomotive designs, T1s were known to be difficult to maintain. The T1s were also so powerful that they could easily exceed their designed load and speed limitations, which in turn often caused wear and tear issues. A technician charged with determining the cause of frequent poppet valve failures on the T1s claimed to have observed them being operated at speeds of up to 140 mph (225 km/h) between station stops in order to make up time on trains that had been delayed. The T1 was designed to run reliably at speeds of up to 100 mph (160 km/h). Although such reports are viewed as rather dubious, it is generally accepted that T1s regularly exceeded 100 mph (161 km/h) in day-to-day operation, making them among the fastest steam locomotives ever built. The price paid for such excessive speed was higher maintenance costs and increased failures in service.

The T-1 4-4-4-4 had such power that the engineer, if not careful on the throttle, could have violent wheel slip at 100 mph (160 km/h), causing damage to the poppet valves. They were described as "free steaming", meaning they could maintain boiler pressure regardless of throttle setting.