Generally you do not have to change the design to fix the problem. Just find out how the loco/tender/track have been damaged and correct them. Bringing them back to original specification will usually fix the problem.
Are the drawbars twisted? They should be parallel to the rail heads when looking straight down the track from the front or the rear. Are the shoulder screws that hold the drawbar to the loco chassis bent?
Many people are rough when removing a steam loco from the track, especially ones with link couplers, and that can damage/twist/bend the drawbar. The drawbar has to be straight and not twisted. The tender truck sides have to be straight and not twisted. The track rail head has to be flat with no dips/bumps and the gauge has to be consistent.
Redesigning may seem like the fix at the moment, but often will introduce new problems. These locos are over 60 years old and thousands have been made and they all worked at one time.
Can you get your eyes down to track level and see what is happening there?
