replacement couplers on lionel el capitain streamliners

If you pop the roof off the car, I believe that gives access to the truck mounting screw.  If you undo that you should be able to get access to the coupler screw.  I think I used some loctite on mine as they had the same problem running on my christmas display.  It's been a year or two since I had them apart to know for sure, but I know you get access to the screws by removing the roof.  Whether or not you have to pull the interior out, I can't remember (I don't believe that was the case though).

To remove the roof I believe is two screws underneath (you should be able to see the mounting posts for them through the windows to help locate the screws).  I believe you have to squeeze in on the edges of the roof as there are lock tabs that hold it to the walls when the screws are out.  As I recall, I usually work from one end to the other to release those tabs and remove the roof.