I don’t know if this was mentioned before since I am new to this forum but if you have stewart RS-3 units the entire drive from the FA-2 loco’s fits perfect in the RS-3 and give you the correct trucks rather than the athearn trucks , you may also use just the trucks and retain the athearn drive , I used the entire drive from a few ruined brush painted loco’s off of ebay , I wanted them the run with the PK-2 units , this holds true for using the motors in EMD units by athearn , the truck gearing is the same and is also interchangable , even on the older AS-16’s , by stewart . this makes a world of difference in the loco’s and the trucks can be purchased direct from lifelike for 7 dollars a pair .
FYI; the “new” “improved” drive in the MDC RS3 is the same as the P2K FA2.
Quite a few years back I was looking for the box cab shells that MDC put out and the factory at that time was in southern calif about 30 minutes from me so I went out there and you could purchase direct from them , in a display case they had the beginning model of the RS-3 using the original RS-3 frame with the openings cut out and brass bolsters across epoxied in place and the new drive installed and the flywheels from the PK-2 drives and the trucks were a dead giveaway , since they have moved to Arizona .