derailments

Hey guys I hope I can find some expertise here. We have a narrow gauge railroad on our park here. It is 24" and used to transport people for a fee around our park. We have had 3 derailments in the last month. All different locations. But on curves. The gauge on the tracks varies from 24" exactly to as much as 24 3/8". We have determined in some of the derailments that a joint misalignment probably cause it. The latest one was on a slight curve. the track appears to be no more than the 24 3/8", train lightly loaded ( 6 passengers) only pulling 2 cars, no joint issues. I am wondering if the under carriage can have a problem contributing to these issue. We think we do a reasonable maintenance job on the cars. They have new wheels and greased regularly.

Any help on this to get me looking in the right direction would be helpful....Don