I've encountered this same split axle problem with a 4-8-4 steamer I have that had not been run for many years. From what I've gathered, the nylon used for the axle dries out over time and splits are very common. It was a poor design, a design isn't used anymore.
From everything I've tried and read, there is no easy fix for a split axle:   
Bachmann no longer carries replacement axles. Glues don't hold for very long. I don't know about how well the brass tubing works because I'm not that gifted when it comes to tooling. I even tried a plastic shrink sleeve method I saw on You Tube, but the sleeve wouldn't stick to the nylon.
I've heard that someone may start printing 3D copies of these axles and offering them for sale; that may be a future solution to the problem. Until that happens, those old locomotives will probably remain static models.
Since I was an original owner, I returned my locomotive to Bachmann for repair. I'm waiting to see how that goes. 

From looking at the forum I know this is an issue that has been discussed before, but not for the last few years.
I have an old Northern 4-8-4 Santa Fe steamer from back in the 1980's (item #660). It hadn't been used for years, but I pulled it out of storage and gave it a try. It worked fine for a short time, but then the wheels stopped turning. I opened it up and guess what? The nylon axle from the rear driver (the one with the gear attached to it) was cracked. I guess this is a common problem with these locomotives.
Apparently, Bachmann no longer stocks this part and glues don't really work to fix the crack. On Youtube I saw a guy who used shrink sleeving and had success, so I tried it and the sleeve wouldn't stick to the nylon. The brass tubing solution sounds way beyond my ability. Has anyone come up with another solution over the years?
If I send it in to Bachmann could it be fixed? I purchased the locomotive new but I have no receipt, so I wouldn't expect it to be covered under warranty. I really like this model since it would work well with my new Santa Fe layout, so I'd be willing to pay a reasonable amount to have it fixed. Or would paying to have it fixed by Bachmann or someone else just be a waste of money?
Thanks for your help.   

I have an old model Overland 4-8-4 that needs lubrication. I can't figure out how to remove the shell - I've found and removed one screw that was behind the trailing truck, but I can't find any similar screw anywhere near the leading truck. Apparently with some older models there was a screw underneath one of the domes, but with this model it appears to be one solid piece of plastic. I've gone over this model with a strong light and a magnifying glass, but I'm totally stumped!