A few months ago I converted a K-Line Berkshire to PS3 for a friend. It ran ok but was jerky below 20 or so scale MPH.
The guy I converted it for was never totally satisfied with the performance. The basic problem is the small forward facing RS385 motor K-Line used. You can see it cogging in the above video. So several weeks ago I was discussing this with @harmonyards and apparently him and @Norton had been discussing swapping out the K-Line chassis for a Lionel chassis used when Lionel reissued these locomotives several years ago. So after talking with Pat I convinced the owner of the K-Line Berkshire to order a chassis and worm gear from Lionel for the Lima Berkshire. I got both the chassis and locomotive from the owner and a 9433 Pittman motor and flywheel from Pat. I set about combining the K-Line Berkshire with the Lionel chassis.
The chassis needed a couple mods. A notch had to be cut in the rear for the PS3 tether connector and holes drilled to mount the connector. In addition a hole to mount the K-Line drawbar had to be drilled. After that a test to see how much the performance improved was in order. Below is the locomotive running at 1 scale MPH effortlessly. Don't worry I cleaned up the wiring and trimmed the tach tape on the flywheel before reassembly 😉.
I reassembled and tested it on a train of heavy scale cars. It runs as well as any PS3 or Legacy locomotive. Nice and smooth. It's a night and day difference from the original K-Line motor and chassis.
So if you've got a K-Line Berkshire this is a worthwhile modification. It vastly improves the locomotive.