I've been inactive for a while, and haven't purchased any Atlas freight cars recently, but Micro-Trains used to include a kingpin that fit into Atlas cars in each package of trucks, and in a parts kit.
I hope this helps.
Dan
I have been looking for the King pins that hold the trucks on Atlas rolling stock, and everywhere I look they are out of stock and have been for over a year. Is there some other way to hold the trucks on, maybe using a screw or something? Thanks, Mike