I only make it to some of the 470 work sessions, so I may not be correct here. Most of the car's structure is original, and in extremely good condition. I think the side sills are fir, but the center may be oak. We were trying to nail into them this past weekend and it wasn't easy.
You might ask how is it that the structure in good shape when the car looked pretty rough in Chama. I attribute that to two things. First it had a good roof on it for most of its life, even though that is one thing that *has* been completely replaced. The second is that is was off its trucks for a long time. If you look at cars the have been restored on the C&TS the ones that were on trucks all their life usually needed all longitudinal sills replaced. Those which were supported in multiple locations, usually did not deteriorate as much.
Bill Kepner