The progression of PRR Steam essentially came to a halt after the M1 because of electrification.
Two reasons behind this:
1. When the PRR started the electrification of the NYC to Phila. main in the late 20s, they displaced their passenger steam locomotives (mostly K4s) from these lines. This surplus of mainline locomotives on the rest of the system diminished the need for new steam locomotives. More and more steam was displaced as the PRR spread electrification from Phila. to D.C. and later extended from Paoli to Harrisburg. There were so many surplus steam engines that they could doublehead on heavy trains (and the PRR was a big fan of doubleheading to their own detriment). There was essentially no need for new locomotives.
2. Electrification became the big focus for the PRR. In between a period of about 10 years from ~1924-1934, they designed and tested 7 classes of mainline electric locomotives (DD2, O1, P5, GG1, R1, L5, L6)***. Research money was essentially being redirected away from steam, and the PRR liked to design in house so it would be unlikely that they would buy a standard design from Baldwin or Lima.
***As a side note, the O1 (4-4-4) was designed to be the electric equivalent of the E6 while the P5 (4-6-4) was designed to be the equivalent of the K4... performance of the locomotives was not as good as the PRR desired on longer trains therefore the O1 was mostly used on shorter (non priority) trains. The subpar performance of the P5 sent the PRR back to the drawing board (leading to the GG1). The P5 was re-geared for freight service once the GG1s were designed.
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By the late 30s when the PRR realized that the aging fleet of K4s needed replacement, steam design had progressed past 4 sets of drive wheels being connected by a single side rod. The reciprocating mass of the side-rod would pound the track. The PRR instead chose to go with an articulated locomotives due to this reason (less centralized reciprocating mass), along with the fact that duplexes could use smaller cylinders, and required lower piston axial force than a conventional locomotive.
Therefore, instead of going with a 4-8-4, they went with a 4-4-4-4 (two sets of 4 drive wheels instead of one set of 8).
During the war, the government did not allow for experimental locomotive design, therefore the J1s were built from the C&O 2-10-4 design as they were needed heavier locomotives for long slow hauls (therefore no 4-8-4 freight engines for the PRR)
The S2 turbine (1944) was supposed to be a 4-8-4 design, however wartime restrictions forced the PRR to use heavier steel alloys in the locomotive (lighter alloys of steel were needed for the war effort). Therefore 6 wheel leading and trailing trucks were used.
By the late 40s, dieselization crept in and there was no need for more new steam locomotives.
