Dennis, as I remember that loco is from the era when Lionel first switched from Pullmor to can motors.
For years going back to the 1940s, Lionel had used a triple-start worm on the big, slow-turning Pullmor motor, yielding a ratio of about 8 to 1. To get more torque out of the smaller motors in the FTs, they substituted a two-start worm yielding a ratio of about 12 to 1. (I think they also used this worm in the FM H-12-44 switchers.) The problem was that on the FTs at least, the mesh wasn't exactly right.
I remember reading that service centers, or perhaps even the Lionel factory, were putting pieces of cardboard(!) between the motor mount and the truck block in an attempt to correct the mesh. Your post is not the first about these double-start worms failing in service. At least one other Forumite ended up replacing his motor and the truck block with a traditional triple-start worm and compatible 25-tooth worm wheel. If you do this, the loco will run faster. But at least it will run.
If the motor recommended by Vernon proves to be a match, pay careful attention to the gear mesh for best operation and longevity. My $.02.
!
