Atlas O screw for truck.

David - Although I do not know the exact size of the screw you are inquiring about, I did lose the same kind of screw off one of an Atlas O pulpwood cars.  I took the screw from the other set of trucks to a local hardware store ( one of the old fashioned kind ... not the big box chain kind ) and they gave me an exact match.  I came home screwed on the set of trucks and the car has been running great ever since.  

there's at least 2 different kinds for atlas cars....the 25k tank cars use a fine thread  machine screw,while most of the others use a course thread self tapping screw...its best to do what trumptrain suggested.

Drag your track slowly with a strong magnet. You will be surprised what turns up. Nick

Wouldn’t it be simpler and more reliable to ask AtlasO service? One of the screws on my Pennsy quad hopper broke apart. I’m going to check with them tomorrow. Or am I naive in thinking that they’ll have the answer?

The fine thread screw is a 2.5mm by 8 mm.  

I've learned the hard way to "bracket" my screw purchases. So I ordered M2.5 in 6mm 8mm and 10mm lengths. I found the 6mm had the least wobble once tightened, although there was still some wobble.

Take the screw off the other wheel set and bring that to the hardware store.

The tank car was an online purchase that came without trucks or screws. I found the screw info here in the forum and ordered screws on faith. Hence the bracketing. I couldn’t find a retailer near me with metric down to M2.5. Hardware stores and big box only went down to M3. And train shops are getting harder to come by :-(

So thanks to Grainger I have a few hundred now and never half to solve this problem again :-)

Got it.