I heard that Lionel bought the molds for the Weaver stuff made in the USA and Atlas bought the molds for the ones made in China. For the locomotives, I think the ones made in the USA were the RS3, FA2, FB2 and GP38. The original ones with the single motor (wonderful) drives were totally made in the USA. As far as I know, when they went to dual motor mechanisms and stamped steel frames, the mechanisms were done in China. I think they mechs were shipped here and assembled. For the freight cars, I think this is what Lionel is now calling the Lionscale line.
I have no idea where the molds for the SD40 were. I am pretty sure the RS11 and U25B were totally made in china. I have a set of the RS11s and had the U25s, and I think the boxes state "made in china".
the higher end freight and passenger cars would be the stuff that Atlas bought. the exception I think might be the B60 baggage which Lionel seems to be building, but maybe the contracted with Lionel or the chinese maker for these. The H30 hopper, war emergency gondola, B&O Wagon Top, Milw rib-side box, and Troop cars were part of what Atlas got.