I recently replaced a damaged non driving wheel set on my Connie. Loco worked OK without any cars attached, although it didn't appear the loco was pulling much current. When I attached several heavy cars the loco wasn't able to pull these cars and again no indication it was drawing any current. Any idea what is wrong?

Hello Everyone,
I have 2 2-8-0 Connies, one recently purchased from Gold Coast Station which has metal gears. The other has been repaired by Barry Olsen which include replacing the drive motor. Both were running very good until recently. I made sure my track was good and clean including using a swifter wet mop to get the gunk off the track. The very same problem with both engines just happened. Both engines will go in reverse just fine. Yet neither of these engines will go forward.
     What are the odds of this problem happening to both engines at the same time? Does anyone have a clue as to what happened.?
Thanks for any help.
Dave