"How long these hoppers (or any other freight car) remained in service depended on the Association of American Railroads (AAR) 40 year rule. It barred cars from interchange service when their under frames reach that age."
The "40 Year Underframe Rule" was implemented by the FRA in 1982, not the AAR. The original rule applied to cars built prior to 1974.
"Since "USRA" hoppers were built from the late 'teens and into the 1920's, they could be in service into the 1950's. Of course that means the cars had been reconditioned on a regular basis to be serviceable over that time, and often had updates, such as installation of power hand brakes that replaced the vertical staff type."
USRA hopper cars were allowed in interchange service during the entire scope of this list as long as they met all the provisions of Rule 3.
"B&O did find a way around that AAR 40 year rule with their M-15 class double-sheathed wood body box cars dating from the mid-1910's. From 1934 to 1936, B&O rebuilt about 1,250 of these M-15 cars with new, steel wagon top bodies on the original M-15 deep, fish-belly center sill under frames."
Again, the underframes of the M-15 class were not subject to any restrictions for interchange, or use in rebuilding programs during the scope of this list. The only requirement for underframes used in rebuilding programs was that the center sill cross sectional area had to be a minimum of 24 square inches on or after July 1, 1928. 1932 ARA center sills were exempt from this requirement as were all tank cars.
"In 1955-1956 these M-15 cars, with their under frames now approaching the 40 year mark, were provided with new AAR design under-frames as they were overhauled."
Simply a rebuilding choice by the B&O, nothing to do with interchange restrictions.