Wood cars lasted through the 1970's in work train service. Most of these cars had steel underframes, however, and were not all wood.
All-wood passenger cars were prohibited from interchange service around 1910 or so, around the time that all-steel cars began entering the national passenger car fleet. If specialized cars such as diners were to continue in use, laws required that they be upgraded with steel underframes.
By the 1930's, few wood bodied cars remained in main line, Class 1 service. Commuter lines and branches continued to use them. Plenty of photos can be found from the 1930's which show 1870-1890-built coaches (open platforms, large spaces between the windows) in work train service. These cars did not usually survive in the post WWII years, when they were replaced by the afore-mentioned wood body, steel underframe cars which were downgraded to work train service.
