I have a Lionel PS-4 flat car that has the trucks below. It seems that one of the little tiny rotating bearing caps is missing and Lionel doesn't show any in stock. If you happen to have an extra, let me know.

I do, somewhere.
I'll have to search my parts.
Send me an email, its in my profile.

I had a pair with couplers that didnt work right. Pretty much destroyed the trucks trying to fix the couplers.

Ironically roller bearings reduced friction, but these actually need to be adjusted and lubed, else the cars don't roll as nice as the models with friction roller bearings...

Last week I received a NIB Atlas Trainman boxcar, and two of the roller bearing caps were loose in the box. I was able to press them on with a pair of water pump pliers. I checked some of my other Atlas cars, and a bearing cap was about to fall off one of them. If it had fallen off, they're so small I don't think I could have found it on my layout.

I have noticed them falling off esp in cold temperatures. Add a bit of Walthers goo at the end of the axle b4 placing them back on. I use goo becasue if you have to get it off, goo gone takes off the goo without destrying the plastic piece.

If you still need one, drop me an email with your mailing address and I'll send you one.
Chuck