. revenue ton-miles are calculated by multiplying the weight of freight by the number of tariff miles . gross and revenue-ton-miles decreased 17% ( 17 % ) and 15% ( 15 % ) in 2009 compared to 2008 due to a 16% ( 16 % ) decrease in carloads . commodity mix changes ( notably automotive shipments , which were 30% ( 30 % ) lower in 2009 compared to 2008 ) drove the difference in declines between gross ton-miles and revenue ton-miles