This ties in with my revision to the esch cummins act seen here
.1927: the PRR hires a Milanese Mechanical Engineer named Augusto Lombardi. Having seen the NYC Hudsons, they desire bigger and better engines, albeit still with interchangeable parts, which they require Lombardi to use on his first designs.1928: Lombardi designs the U1 2-8-4, based on the I1. And the R2 4-8-4, based on the M1. Both are moderate successes. But are not built in quantity like other PRR steamers of the time. For this reason of the lack of interchangeable parts, Lombardi is given permission to make entirely new designs.1929: Lombardi introduces a new method of locomotive numbering due to numbering system having once been random. The process dictates a letter for the motive power type (S for steam, E for electric, etc.), the class of the engine (L, K, M, I, etc.), and the engine’s number. For instance, L1 Mikado 520 would be referred to as “SL-520.” and K4 Pacific 1361 would be “SK-1361"1930: After a few years, Lombardi announce that he will work with Baldwin and Lima to develop his next big project. A larger engine to replace the K4 Pacific in fast Passenger runs.1933: SR-7100, the first R3 4-8-4, rolls out of Altoona. 300 would be originally built with the intent of replacing the K4 Pacific. However, they would instead supplement them often and end up on mixed traffic later in life.1935: The first U2 2-8-4, SU-7400 rolls out of Altoona. The railroad envisions these engines to compliment the L1 Mikados and M1 Mountains in fast freight. With I1s and U1s roll on heavier, slower trains.Late 30s: Lombardi builds the R3s and U2s in massive numbers. Unlike the past PRR engines, they have fewer interchangeable parts. but are still loved by crews and the PRR.1938: The electrification of the NE Corridor is completed. Though some engines continue to venture as far as Harrisburg.1940: Many prominent members of the Norfolk and Western brass begin replacing their PRR counterparts in preparation for merger. Which doesn't happen until 1978. Most significantly, William J. Jenks, president of the N&W, becomes a key advisor of Martin W. Clement of the PRR. 1942: The PRR introduces the first two T1 duplexes. Along with them, they also build some streamlined 4-8-4s called R4s to serve as backup.1945: The PRR builds the J1, T1, and Q2 engines.1946-1950: The PRR begins modifying their facilities to be like those of is subsidiary Norfolk and Western. Allowing for a more efficient maintenance process than before. In addition, several more N&W J class steamers are built for use on the Pennsy. Under which they becomes the R5 class of engines. As such, engine like 611 or the fictional 622 become SR-611.
Many more N&W types are built at Altoona and used on both railroads.1957: The first K4, 1737, is retired and put on display at Northumberland, PA.1957-1963: Most of the K4 Pacifics are retired. The good news is that among the ones preserved in this timeline are 5399 (my personal favorite of the class), 3768 (with rebuilt shrouding), 1120 (also with rebuilt shrouding), and 3847. Plus many more K4 Pacifics. Which become to Americans what the Black Five is to British enthusiasts.1965: All PRR steam engines smaller than the M1 Mountains are retired. This includes the L1 Mikados, H series Consolidations, among others. Only the I1 2-10-0s and several switchers remain by 1976.1966: The R2, U1, Q2, and T1 are all retired. Thankfully, several are preserved of each class.1967: The PRR introduces the Keystone Limited, which supplements the Broadway Limited as a NY-Chicago train, albeit one running via the Panhandle Line through Columbus, OH and Logansport, IN.
Steam continues well until the 1980s. At which point the fleet is mainly R3s, I1s, M1s, and N&W engines. Plus a few K4s in preservation and L1s on branch line traffic.