Just to add additional unspecific info, I'm not sure there ever was a rule "banning" wood underframe cabooses that used such language.? But around 1908 or so -- others may have specifics -- after a serious wreck with a four-wheel caboose the ICC proposed a ruling with wording similar to: All cabooses must be of a weight similar to, and must be able to withstand the buff forces from, the rolling stock currently in service, etc., etc., etc.?I'm not?near my library (home computer is dead), but I think the gradual phasing out had to be complete in the early 1920s.?Four-wheel and wood-underframe cabooses were the victims, though most railroads had their own policies on using them in MOW and engineering department service, some surviving quite a while.? Some states got into the act, as New York had its own law of similar language, which is the event that I read about?somewhere that started the Pennsy's phasing out of bobbers.

Mike Del Vecchio