It would have to be the Pullman-Standard 50' box car with exterior bracing that has the flat roof which was built during the 1970's and the 1980's. That P-S box car has never been produced in 1/48th scale.
They need to make a classic, distinct box car which can be produced in many actual railroad schemes from the 1970's and the 1980's, before all the huge mergers.
Andrew.