Mobile Mail-Screening Station
A Mobile Mail-Screening Station is a highly specialized, self-propelled facility designed to screen mail for the presence of explosive devices and to deter people from sending chemical, biological, radiological and nuclear weapons through the mail, specifically to certain special events. Mobile Mail-Screening Stations are operated by the United States Postal Inspection Service.
Source: Wikipedia — Mobile Mail-Screening Station (CC BY-SA 4.0)