At the beginning of the 30s. The PRR had literally no northerns. But by the time the decision to completely abandon steam in the early 70s. There were a total of seven classes of Northers. All in the R system, they and their histories are as follows.
The R2: Originally built simply as a larger version of the M1s Mountain. These mainly served slower passenger trains and fast freight.
The R3: the most numerous of the PRR’s Northern classes. This engine served alongside the T1 duplexes and diesels on fast passenger trains. Though others ran on fast freight. Based on creation of o484, who made more versions of the R3 idea.
The R4: These were built as lighter and smaller versions of the Q2 4-4-6-4 duplexes. They primarily served as coal haulers on the line from Erie to Harrisburg.
The R5: These were originally intended to serve as replacements for the T1s. But only a few were built. Due to a series of experiments which managed to fix most of the T1’s problems. The five that were built were put on service as fast freight engines on the Panhandle line.