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正常句子语序是:They argue,if railroads charged all customers the same average rate,shippers who have the option of switching to trucks or other forms of transportation would do so, leaving remaining customers to shoulder the cost of keeping up the line。
主句的主干是shippers…would do so, 其中who引导的定语从句修饰shippers。最后一个逗号的后面有两个现在分词，但是用法不同：leaving sb. to do sth.充当状语;而remaining作customers的定语，可以译为“剩余的客户”。