I repaired my Lionel 6-11333 ATSF Northern 4-8-4 #3759 that would not run forward. I found that Lionel put a Universal Joint shaft (Plastic Dog Bone) that was 3/16" too short in the engine. The shaft the Chinese put in the engine was just long enough to work when new out of the orange box, but would wear the T-shaped arms and round them, so the shaft would not engage and just make clicking sounds.
The next engine for me to repair will be the Lionel 6-82768 Legacy Chesapeake and Ohio Allegheny #1604 that Lionel Service did not properly fix. The engine jerks down the track between speed step 22 and 110. The person who serviced it at Lionel, no longer works there. Wish me luck.
Sincerely, John Rowlen