+1 on Brian's recommendation to explore rpyn's Interchange forum. I just did a search with the terms "wood" and "water" and found a bunch of relevant posts. It is clear to me that there's much prior art in mitigating water in this domain.
For what it's worth discussing here, I'll throw out the following:
1. I don't think it's enough to just "rework-to-print" the existing installation. Some design to insure water doesn't get to places where it can do damage is in order.
2. However, water-resistant design may run counter to the preservation of historical integrity, but if that's the priority then an interior or at least a covered display venue may be the only way to insure long-term integrity of the artifact.
3. In any event, relying on periodic maintenance of sealant and paint may be problematic if the funds cannot be assembled to support this, which lends priority to repairing with a water-resistant design. If you can't afford to paint it, you probably can't afford a shelter, so rework it to not rely on those things.
4. The original design and construction of the car may have considered water incursion, and the repairs may have lost those considerations. From Jason's OP: "What took decades to happen with the wood used on the car 80 to 120 years ago has taken less than a decade and a half to happen to the wood used on the early this century." This may not be just because of the quality of the wood. I hesitate to bring this up, knowing that competent and well-intentioned folks did the restorative reworks, but I've seen this in the aerospace industry where really good techs and engineers attempt to fix something without benefit of the engineering artifacts (drawings, procedures, etc) that comprise the "build paper". It never ends well...
Geesh, I'm just a long-time lurker. And, I'm not experienced in railcar restoration. So, take this for what it's worth, just one fellow's (possibly ill-considered) observations...
