How do you change trucks on a Lionel O Gauge tank car? Looks like the screw holding the truck is on the inside of the tank car.
Welcome to the Straight Dope Message Board!
As this is a “general interest” message board, you may not have a lot of luck finding a good answer to your question here, unless one of our board members happens to be a Lionel enthusiast (which is entirely possible). You may have better luck posting your question on a message board that’s specifically devoted to model trains and toy trains.
I’m not expert on the subject (though I have some Lionel trains), but here are a couple of boards that look promising:
https://ogrforum.ogaugerr.com/
Good luck!