Pennsylvania Railroad 5550 (PRR 5550) is a mainline duplex drive steam locomotive under construction in the United States.

This was due to its unusual duplex drive 4-4-4-4 wheel arrangement, its use of the Franklin Type A oscillating-cam poppet valve, and its characteristic streamlining conceived by renowned industrial designer Raymond Loewy.

[5][1] The T1 was also the only class of Pennsylvania duplex able to travel the railroad's entire network and the first production series locomotive designed to use the poppet valve.

[1] The original materials used to construct the valves were subject to fatigue issues, particularly when the locomotives were operated above 100 miles an hour.

[1] Despite these issues, the poppet valves did improve the T1's high-speed performance, requiring less horsepower and distributing steam flow with greater precision.

[10] The T1 locomotives were further burdened by excessive wheel slip on one of the two engine sets at startup or high speed.

[5] However, no complete solution was found to the problem of wheel slip, even though an "anti-slip" mechanism had been previously installed on the PRR Q2-class duplex.

[5] Before many of the problems that plagued the T1 class could be solved, the Pennsylvania Railroad decided to begin the transition from steam power to diesel.

The T1 Trust is composed of railroading experts who intend to build the 53rd member of the T1 class and put it into mainline steam excursion service within the United States.

[5] The builders of PRR 5550 hope to break the world steam speed record, which is held by the LNER Class A4 4468 Mallard at 126 mph (203 km/h).

[11][13] The chairman of the T1 Trust is Bradford Noble, with Scott McGill the chief mechanical officer, Wes Camp the director of operations, and Jason Johnson the general manager.

[8] The digitally-scanned drawings collected by the T1 Trust are used to create detailed and intricate 3D Models using the Dassault Systèmes Solidworks computer aided drafting (CAD) program.

[8] Construction of PRR 5550 was officially started on May 31, 2014, with the completion of the locomotive's bronze keystone-shaped number plate, following a successful conclusion to the initial Kickstarter campaign launched by the Trust.

On May 2, 2014, the T1 Trust began meetings and discussions with the Federal Railroad Administration to help ensure that PRR 5550 is constructed to FRA standards.

Franklin Type B gear was applied to a handful of locomotives starting in 1948, including K4s 3847, which had been designed or built with conventional piston valves.

5500, perhaps the first locomotive fitted with two types of poppet valve gear, was soon noted for its superior performance over the other engines of its class.

[5] It is worth noting the Pennsylvania Railroad also experimented with the idea of fitting the conventional Walschaerts valve gear to the T1 and retrofitted number 5547 to such a configuration.

USATC 611 was fitted with Franklin Type D rotary-cam poppet valves during the 1950s during its career at Fort Eustis, Virginia.

[11] On July 8, 2015, the T1 Trust launched a Kickstarter campaign titled "Let's Get Rolling" to raise $20,000, to be used in the design and building of a casting pattern for the new Boxpok drivers.

[5] This historic event marked the first time a steam locomotive driving wheel had been cast in the United States since the 1940s.

By coincidence, the facility currently housing Curry Rail Services was previously the Pennsylvania Railroad's Samuel Rea Car Shops.

[8] Construction on the "prow" portion of the locomotive's streamlining, also made of aluminum, began on November 10, 2016, by Gemini Industrial Machine.

Using existing CAD models, a water jet cutter, bodywork, fabrication and paint, the prow was completed on May 16, 2017, with the headlight and keystone number plate attached.

[18]: 28:59  The 5550 frame is designed to have a buff strength exceeding FRA requirements for durability and running at high speed on 4 cylinders.

Others working on the design of the boiler included Trust members Gary Bensman, Dave Griner, Scott McGill and Jason Johnson.

The motor, coal crusher, gearbox, stoker trough, and reversing valve were donated by Gary Bensman and Warren Lathom.