I have a number of those early MPC cars with the AAR trucks as well. I suspect that the tooling may not have been worn out per se, (the tooling was less than a decade old and molds used for plastics tend to last longer than those used for die-cast) but perhaps when Lionel modified it for the new needlepoint axles surfaces in the tooling got roughed up somewhat in the process, or perhaps it was an early production problem with the delrin plastic combined with the injection molding process that resulted in the trucks looking a bit rough and the oxidization on the trucks; usually the residue comes off with a shot of Pledge and a rag.
Needless to say, I really wish they fixed that early problem with the plastic trucks and kept using those carryover postwar AAR trucks or used a new truck design similar to the ASF ride-control type type trucks LTI came out with after 1989 instead of those very obscure Symington-Wayne trucks.
The AAR truck tooling was actually in use about 13 years (starting in 1957) before the MPC guys came in, so it's not completely out of the question that it could have been worn out. But I kind of agree with John, there might be more to it than that as the AAR trucks on 1969 production don't seem to be anywhere near as bad as the ones found in the early 1970's. While I doubt that it could have been screwed-up while making the modifications in the early 1970's (the outside surface of the sideframe is on a completely different tooling surface that likely wasn't touched to make the mods), but the potential use of a different plastic has some merit. Different plastics have different properties -- shrink rates being among them -- and that could also result in some of the truck's rough appearance from this time period. If I had to bet, it's probably a combination of all of the above.
As for the choice of the oddball Symington Wayne XL-70 trucks, the thoughts on selecting that particular design might be lost to time. I even tracked down the original Lionel designer of the truck, but he couldn't recall anything other than he used the material that someone gave him.
The F3s that were previously noted should have come equipped with horns as MPC still included those. During the postwar period Lionel originally used a two-piece horn assembly, but during the aforementioned cost-cutting measures in the mid- late-50s they started using the same one-piece horns that were originally used on the postwar Alcos and Geeps and this along with the other detail-removals simply carried over to the MPC era, exception that MPC took the additional step of removing the molded-in front nose grills.
I think the poster was referring to operating horns, but between our two posts, I think we covered the bases!
