First,Pennsylvania Railroad tried a Norfolk and Western Northern Type J-Class 603 on the line to Chicago and then turned down the 4-8-4 any info why Pennsy decided to turn down the 4-8-4.
I have always assumed that the problem was tat PRR's top people had fallen in love with the duplex design. And maybe the N&W J didn't impress them as much as it has impressed many people. I know the N&W engine was capable of high speeds despite its comparatively low drivers, but N&W's routes suggest that it didn't, in service, have to do as much sustained high-speed running as a passenger engine on PRR's Fort Wayne division would have had to.
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Of course, the PRR did have a 4-8-4: the R-1. (Grin!)