I would suggest calling Atlas CS (customer service) and ask them what size it is. They must use the same size screw for more than just the loco You have, past/present. Calling is probably quicker.
http://shop.atlasrr.com/contactus.aspx
I'm in HO, so I would just be guessing, with a size.
Take Care!
Frank
Try the Atlas internet site. They list a lot of parts maybe your screw is listed.
Joe
My Loco lost the screw holding the rear truck on.. and the drivers "jumped time". !
Can anyone PLEASE tell me what size screw I need to buy?
and better yet- where to get it ?
( I tried to "borrow" the shell mount screw from the top - it's different)
Thank You ! -Pete