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A Norfolk Southern freight train rumbles through Luray, Virginia February, 18, 2012. Freight trains deliver goods to and from the inland port at Front Royal, Virginia.

First a riddle...Why did the train carrying biofuels cross the border 24 times in 13 days and never unload any cargo? Stumped? 

OK. Here's the answer: To earn millions of carbon credits.

John Nicol, an investigative journalist from the Canadian Broadcasting Corporation, is here to explain.

RELATED: Biofuel credits behind mystery cross-border train shipments