Also be very suspicious of “new” brushes. Somewhere along the way Lionel greatly increased the brush resistance, to the point where I no longer regard them as a compatible replacement part. I’m not sure how to tell these new “bad” brushes from the older good ones. If your brushes are worn down and really short, steal the brushes from a vintage junker (a similar donor loco) with low run time. In my experience it takes a lot of running to get to that point.
Also make sure your brush spring tension is correct. This is the darkest of the Dark Arts. Too much and the motor won’t turn freely. Too little you’ll get sparking and intermittent operation. When in doubt replace the whole brush plate with one from a low run-time unit.
When I change brushes, I don’t even mess with the springs. I put the brushes back in their wells from the commutator side to avoid stretching the springs.
It won’t hurt to try the new power truck, that’s good troubleshooting. But personally I would have spent that $30 on a more powerful transformer from the 1950s or 60s. I think your transformer is the culprit!
