The only rule that I know of for express boxcars is that, on railroads that had maximum authorized passenger speeds of 90 to 100 MPH, passenger trains containing "freight equipment" were limited to 70 to 80 MPH. That shouldn't be a problem in O-gauge. 
Express reefers (which actually had simple passenger car trucks) were allowed the same maximum speed as other heavyweight ("Madison") cars. On some roads, only lightweight (streamliner) cars were allowed the full maximum passenger train speed.
