Your postal carrier likely drives a truck run on a fuel called biodiesel. It's an alternative fuel used by many government and municipal vehicle fleets. The Soybean oil additive is said to help engines run cleaner. It's better for the environment, and could boost the price of soybeans. The use of biodiesel has tripled in the last four years. But it still accounts for only a tiny fraction of the fuel used in this country. Less than one percent of all the soybean oil is used for biodiesel. In our last installment of the Mainstreet series "The Miracle Bean", Cara Hetland reports it'll take federal subsidies and mandates to help the biodiesel industry become more competitive and more affordable.