When did Lionel switch to DC motors?

Yeah… when did THAT happen?

Lionel experimented with DC motors in their low-end sets beginning in 1973.  By about 1983, everything in their "Traditional" (train set) line was DC motors.  Those tiny motors aren't very good quality, and probably won't give good performance with a RailPro receiver.  A retrofit would be like trying to make a silk purse out of a sow's ear.

One of the first "Collector Series" locos with a can motor was the 1992 Southern Mikado, and a transition gradually follwed.  I would say that can motors in the high-end stuff were the exception rather than the rule until about 1999.  With the move to Korea, all of the better stuff got can motors, and after about 2005, AC "Pullmor" motors were only made in special commemorative reissues.

Hope this helps.  If you do a RailPro install, please post some pics or videos, let us know how you think it performs.

DC can motor quality/longevity depends on the quality of the permanent magnet. IMO, the magnets have improved. 

@Mad_Liver posted:

I am in the process of adding railpro to my locos and was wondering when Lionel made the switch from AC to DC motors?

For clarification: although Lionel switched to DC motors in some of their locomotives, they have internal circuitry that converted AC from the transformer to DC for the motor. Basically a rectifier with some supporting electronic components. DC motors are cheaper to produce and more readily available, hence the switch to DC motors.

Steve