No. 6110, the "sister" prototype of class T1 prototype No. 6111. Its streamlined casing was designed by renowned industrial designer Raymond Loewy. Click to enlarge.

(Wilkes-Barre Record, Public domain, via Wikimedia Commons)



The Pennsylvania Railroad (PRR) Class T1 duplex-drive 4-4-4-4 steam locomotives, introduced in 1942 with two prototypes and later in 1945-1946 with 50 production examples, 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 wheelslip both when starting and at speed, in addition to being 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 units.

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 stiffness 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 wheelslip problem.


A T1 prototype leaving Chicago's Union Station in February 1943 with the Manhattan Limited to New York.

(Jack Delano, Public domain, via Wikimedia Commons)



Before the T1, the last production express passenger engine the PRR had produced was the K4s of 1914, produced until 1928. Two experimental enlarged K5 locomotives were produced in 1929, but they weren't considered enough of an improvement to be worthwhile. After that, the PRR's attention switched to electrification and the production of electric locomotives; apparently, the railroad decided that it did not need more steam locomotives.

However, the deficiencies of the K4s became more evident during the 1930s. The locomotives performed well, but as train lengths increased they proved to be underpowered; double headed K4s locomotives became the norm on many trains. The railroad had many locomotives available, but paying two crews on two locomotives per train was expensive. Meanwhile, other railroads were leaping ahead, developing increasingly-powerful passenger train locomotives. Rival New York Central built 4-6-4 Hudsons, while other roads developed passenger 4-8-2 "Mountain" type and then 4-8-4 "Northern" type designs. The PRR's steam power began to look outdated.

The PRR began to develop steam locomotives again in the mid-to-late 1930s, but with a difference. Where previous PRR locomotive policy had been conservative, new radical designs took hold. Designers from the Baldwin Locomotive Works, the PRR's longtime development partner, were eager to prove the viability of steam in the face of new competition from Diesel-electric locomotives. They persuaded the railroad to adopt Baldwin's latest idea: the duplex locomotive. This split the locomotive's driving wheels into two sets, each with its own pair of cylinders and rods. Until then, the only locomotives with two sets of drivers were articulated locomotives, but the duplex used one rigid frame. In a duplex design cylinders could be smaller, and the weight of side and main rods could be drastically reduced. Given that the movement of the main rod could not be fully balanced, the duplex design would reduce the "hammer blow" on the track. The lower reciprocating mass meant that higher speeds could be achieved. Use of poppet valves also increased the speed because they gave very accurately timed delivery of steam to the cylinders. However, there was a drawback of the metallurgy used; the poppet valve could not withstand 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 No. 6100 of 1939. It managed to reach 100.97 miles per hour (162.50 km/h) on level track while pulling a 1,350 ton passenger train. Its performance encouraged the PRR to continue to develop duplex steam locomotives. The S1 was built unnecessarily-large for her exhibition at the 1939 New York World's Fair until October 1940; therefore, its turning radius prohibited it from operating over most of the PRR network. The 6-4-4-6 design reduced driving set traction to the point that it was especially prone to wheel slip; thus only one Class S1 was built. The PRR returned to Baldwin to develop a duplex design fit for series production. The PRR ordered two Baldwin prototypes (Nos. 6110 and 6111) at a cost of $600,000 on June 26, 1940. Both prototypes had numerous teething problems and were prone to wheelslip if not handled carefully by the engineer. But favorable test reports resulted in a production order for 50 T1s, split between the PRR's own Altoona Works and Baldwin. On Dec 20, 1944, the PRR Board authorized the purchase of 50 Class T1 locomotives for $14,125,000 ($282,500 per unit). Baldwin's chief designer, Ralph P. Johnson, was responsible for the mechanical aspects of the new T1 class. Designer Raymond Loewy obtained US Patent D 136,260 for an early T1 conceptual design with a high-mounted cab located over the forward driving set. While that suited Baldwin's objective of making the most distinctive steam locomotive possible, practical considerations led the T1 design to be revised to the conventional cab position with a slight modification of the unique nose design included in Loewy's patent.

The last production T1 (no. 5549) entered service on August 27, 1946. Engine No. 5539 developed 5,012 hp (3,737 kW), as tested between September 11, 1946, and September 14, 1946, by Chesapeake and Ohio Railway dynamometer car DM-1 while on loan to C&O. In 1944 no. 6110, tested on the stationary test plant in Altoona, developed 6,550 ihp (4,880 kW) in the cylinders at 85 mph (137 km/h). They also regularly racked up over 8,000 miles a month.

Due to their complexity relative to other steam locomotive designs, the T1s were difficult to maintain. Designed to run reliably at speeds of up to 100 mph (160 km/h), the T1s were so powerful that they could easily exceed their designed load and speed limitations, which in turn caused increased wear and tear, particularly to the 100-mph-limited poppet valves. They were described as "free steaming," meaning they could generally maintain boiler pressure regardless of throttle setting. They were so powerful that violent wheel slip could occur over a wide speed range if the engineer did not handle the throttle carefully; loss of driver traction at high speeds, especially when the T1 was under heavy load while ascending grades, caused damage to the poppet valves. The Franklin Type A valve gear applied to the T1s was designed for continuous speeds at 100 mph, and sprints up to 125 mph. 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 143 mph (230 km/h) to make up time.


Prototype No. 6110 pre-delivery at Baldwin.

(Photographer unknown, Baldwin Locomotive Works, W. Lenheim Collection)



When the PRR Board decided to dieselize all first-class prime trains in 1948, most T1s were downgraded to haul secondary trains. Some of them were withdrawn from passenger service in 1949; all were out of service by 1952. They were all scrapped between 1951 and 1956.


No. 5550

Main article: Pennsylvania Railroad 5550

In 2014, a non-profit group known as The T1 Trust began constructing an all-new, fully-operational T1 using the original plans with subtle performance improvements where necessary. The T1 Trust's goal is to provide mainline excursion service and to set the world speed record for a steam locomotive – currently held by the LNER Class A4 No. 4468 Mallard at 126 mph. The T1 Trust's cost estimate to build T1 number 5550 is $10 million, with an expected completion date of 2030 (This total has since been reduced to a bit more than $7 million, as a used PRR long haul tender has been acquired in lieu of new construction).

The construction of 5550 is also following construction and financing methods pioneered by the LNER Peppercorn Class A1 60163 Tornado project. The first piece of the locomotive, the keystone-shaped number plate, was cast in April, 2014, followed by the first minor component, a driving spring link pin, in October, 2014. Major components completed as of March, 2019 include two Boxpok drivers, the prow, cab, third-course boiler and fire door. Front tube sheet construction was under way by a fabricator in St. Louis, Missouri.


Photo of a Pennsylvania Railroad TR1 locomotive, 22 September 1949. Click to enlarge. (Cowles Syndicate, Public domain, via Wikimedia Commons)

PRR 6110, a PRR T1 4-4-4-4 duplex locomotive, at Chicago Union Station ready to leave with the Manhattan Limited, February 1943. Click to enlarge. (Jack Delano, Public domain, via Wikimedia Commons)


PRR 6110, a PRR T1 4-4-4-4 duplex locomotive, is coupled to the Manhattan Limited at Chicago Union Station. February 1943. Click to enlarge. (Jack Delano, Public domain, via Wikimedia Commons)

In media

A computer-generated version of a T1 was seen in the 2004 film Lemony Snicket's A Series of Unfortunate Events.


Pennsylvania Railroad T1 Overview

Type and origin
Power type: Steam
Designers: Ralph P. Johnson, Raymond Loewy
Builders: Altoona Works (5500–5524), Baldwin Locomotive Works (5525–5549, 6110–6111); Pennsylvania Railroad T1 Steam Locomotive Trust (5550)
Serial numbers: Altoona 4560–4584, BLW 72764–72788 (5525–5519)
Build date: 1942 (6110–6111), 1945–46 (5500–5549), 2014–present (5550)
Total produced: 52
​• Whyte 4-4-4-4
• UIC 2′BB2′
Gauge: 4 ft 8+1⁄2 in (1,435 mm)
Leading diameter: 36 in (914 mm)
Driver diameter: 80 in (2,032 mm)
Trailing diameter: 42 in (1,067 mm)
Wheelbase: 107 ft 0 in (32.61 m)
Length: 122 ft 9+3⁄4 in (37.43 m)
Width: 11 ft 1 in (3.38 m)
Height: 6111: 15 ft 6 in (4.72 m)
Axle load: 71,680 lb (32.51 t)
Adhesive weight: 279,910 lb (127.0 t)
Locomotive weight: 502,200 lb (227.8 t)
Tender weight Empty: 197,400 lb (89.54 t); Loaded: 442,500 lb (200.7 t)
Total weight: 944,700 lb (428.5 t)
Tender type: 180 P 84
Fuel type: Coal
Fuel capacity: 85,200 lb (38.65 t)
Water capacity: 19,200 US gal (73,000 L; 16,000 imp gal)
• Firegrate area 92 sq ft (8.5 m2)
Boiler: 100 in (2,540 mm)
Boiler pressure: 300 lbf/in2 (2.07 MPa)
Heating surface: 5,639 sq ft (523.9 m2)
• Tubes and flues: 4,209 sq ft (391.0 m2)
• Firebox: 490 sq ft (45.5 m2)
​• Heating area: 1,430 sq ft (132.9 m2)
Cylinders: Four
Cylinder size: 19.75 in × 26 in (502 mm × 660 mm)
Valve gear: Franklin poppet
Valve type: Poppet valves
Performance figures
Maximum speed: Over 110 mph (177 km/h)
Power output: 6,500 ihp (4,800 kW)
Tractive effort: 64,653 lbf (287.6 kN) (85%)
Factor of adhesion: 4.33
Operators: Pennsylvania Railroad
Class: T1
Number in class: 52 original, plus 1 under construction
Numbers: 6110, 6111, 5500-5549, 5550
Withdrawn: 1952–1953
Scrapped: 1953–1956
Disposition: All 52 original scrapped, 1 new build (PRR 5550) under construction