But many shippers complain that for heavy bulk commodities traveling long distances, such as coal, chemicals, and grain, trucking is too costly and the railroads therefore have them by the throat.
请教老师for heavy bulk commodities traveling long distances, such as coal, chemicals, and grain,是作状语修饰trucking,其中的traveling long distances 是作后置定语修饰heavy bulk commodities,还是说heavy bulk commodities traveling long distances整个作动名词复合结构呢?