A few years ago, I was fortunate enough to buy a Mainline 56XX at a show, that had been converted to P4 by a well-known professional modelmaker, from the man himself. The loco features a scratchbuilt chassis (with single rocker compensation), Ultrascale wheels and a Portescap motor.
When I first bought it, it ran really smoothly. The chassis needed a little paint re-touching and the body has been waiting for a few additional details, but it is essentially a nice and useful loco.