Old Athearn A.T.&S.F. boxcar, Precision Scale CB&Q converted troop car and Intermountain S.P. boxcar with Allied trucks.
I like to use the MTH R50B cars
Express boxcars run in passenger trains in the steam era needed to have steam lines to get the heat to the passenger cars and they needed a signal air line. They also needed better trucks so as not to overheat at passenger speeds. Finally the external appliances such as grabs had different requirement in passenger service and all the ones I have seen had stirrup steps at the door. So a paint job alone does not an express car make - for a passenger train.
I think the most of the above applied pretty much through until Amtrak.