Regarding standard gauge cars.
Santa Fe steel head-end cars from the 1920s are equiped with doors. Santa Fe RPO #60 the class car built in 1924 has doors on both ends -- this would enable the staff on long runs to move mail to and from adjacent storage mail cars. Santa Fe 2055, a bag-RPO from 1929, a "creep hole" between the two sections located below the U-shaped sorting case.