Check the rims of all the drivers. They come loose between the rim and the white insulator. That makes the loco fall off the track.
Push the rims side to side to see which ones are loose.
If they are loose, clean them with alcohol and epoxy them back into place. This is easier to do with the rods removed so the axle can be rotated by hand. Rotate the wheels to make sure the rim is straight before letting the epoxy set up.
