Frequently the trucks (the wheel/coupler assembly that sits under train cars) come unattached. Lionel trains from the 1970s and first half of the 1980s are especially prone to this, though other makes of trains aren’t immune either. And sometimes you just want to change the trucks–some Lionel and Marx O27 cars are just the right size for American Flyer S scale, for example, only the trucks are the wrong gauge.
It’s tempting to try to just re-attach them with a nut and bolt, but as the train runs in circles around the track, the nut loosens and eventually works its way out.
The key is all in the type of nut you use.
David Farquhar is a computer security professional, entrepreneur, and author. He started his career as a part-time computer technician in 1994, worked his way up to system administrator by 1997, and has specialized in vulnerability management since 2013. He invests in real estate on the side and his hobbies include O gauge trains, baseball cards, and retro computers and video games. A University of Missouri graduate, he holds CISSP and Security+ certifications. He lives in St. Louis with his family.