Had trouble with certain engines coupling with cars on straight sections AND curves .
I began replacing MTH, Lionel and Williams fixed couplers with the MTH articulated couplers .......END of troubles.
This was the last one at the time that I had done sometime ago. It was a Lionel gondola scrap car. I had to insert washers with the screws to limit movement of the screws in the trucks . I applied some Locktite Blue to limit screw movement on the washers.
Have a great weekend at the WORKBENCH. God speed.