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In July, a speeding train killed 77 near Santiago de Compostela, Spain. Earlier in the same month, a freight train carrying crude oil derailed in Lac-Mégantic, Canada, killing 47. And just last May, two commuter trains in Connecticut collided, injuring more than 70. What is it with trains and safety? CNN's Tom Foreman takes a look at what's behind most train wrecks.