I believe the issue of zinc rot is a common one with the JLC scale GG1 locomotives. I recently purchased a NIB #4866 on Ebay and it arrived with the trucks disintegrating to the point that one had broken completely in half during the shipping process. Of course this is one part Lionel no longer has in stock, and thankfully I saw a listing by a forum member shortly before I acquired the locomotive who stated he had them in stock. I bought four, since the GG1 I already had, #4925, was also showing signs of zinc rot upon inspection. An easy but expensive repair, but I'm now happy with my purchase. It's still cheaper than a new Vision Line one.
If you've got one of these you haven't run in awhile, take a look at the condition of the trucks.
The next few photos are post-repair. On my #4925, the gap between the trucks was non-existent due to the swelling.
A video of my new locomotive pulling my set of MTH Congressional cars. #4925 is with Mike Reagan for repair of some operational issues it had prior to the truck rot issue being discovered.