1.1 This specification covers seven classes of carbon steel tires for railway and rapid transit use. 1.1.1 Class A-For untreated driving tires for locomotives in passenger service. 1.1.2 Class AHT-For heat-treated driving tires for locomotives in passenger service. 1.1.3 Class B-For untreated driving tires for freight locomotives and tires for locomotive-truck, tender-truck, trailer and car wheels, and miscellaneous service. 1.1.4 Class BHT-For heat-treated driving tires for freight locomotives and tires for trailer wheels. 1.1.5 Class C-For untreated tires for switching locomotives. 1.1.6 Class CHT-For heat-treated driving tires and switching locomotives and tires for locomotive-trucks, tender-trucks, trailer and car wheels, and miscellaneous service. 1.1.7 Class DHT-For heat-treated driving tires for locomotives with light braking conditions, heavily loaded trailer tires, and rapid transit wheels where off-tread brakes are employed. 1.2 Supplementary requirements, including those in the general requirements of Specification A788/A788M, are provided for use when additional testing or inspection is desired. These shall apply only when specified individually by the purchaser in the order. 1.3 The values stated in either SI units or inch-pound units are to be regarded separately as standard. Within the text and tables the SI units are shown in brackets. The values stated in each system may not be exact equivalents; therefore each system shall be used independently of the other. Combining values from the two systems may result in nonconformance with the standard. 1.4 Unless the order specifies the applicable “M” specification designation, the tires shall be furnished to the inch-pound units. 1.5 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.