I’m probably suggesting things you already have tried but here goes.

It sounds like the coupling you describe won’t pivot at curves so the tender wants to stay straight behind the engine even when going through curves.

My first suggestion would be to lubricate the fork and pin assembly with powdered graphite. Avoid using oils unless you are certain they are compatible with plastics.

If lubrication doesn’t work, you may have to ream out the inside of the fork just enough to reduce the friction but not so much as to over do it. If the fork is made of plastic, insert a narrow hobby knife or file and gently rotate to remove tiny amounts of material, checking the fit & play frequently.

If you prefer (or it is easier), shaving the pin would have the same effect. Just be careful not to take too much off or the coupling will be too loose.

One final thing to try and see if the tender can make it around curves all by itself. If it derails then you know that the tender’s trucks are probably the culprit and not the fork & pin assembly. If this is the case, there are a couple recent tips below this one that can help you address that problem.

Hope this helps,

Geof from the US