Steel underframe, and IIRC, a rather beefy one. Wouldn't want to try to have a conversation, though.
Some CFs rebuilt with plywood sides survived much longer in secondary service. 300+ built all together. Interestingly N&W went from basically a single caboose "family" all based on the CF to an extremely diverse caboose fleet due to mergers. Virtually all of them done in brass by OMI too, which is actually kind of amazing.
