Yes, well that is a no-brainer... oil! But what is hard to understand is why so much of this precious cargo is being lost to accidents during shipping. Follow this link and join in the discussion... or just have a good laugh : ANOTHER CANADIAN OIL SPILL
My guess is most likely it is the infrastructure of the railroad that is the culprit. The trains are becoming more heavy each generation so the old rails and crossings are unable to handle the additional loads. Much like the highways after the maximum gross weight for trucks was raised and the warehouse taxes became too onerous for business so they put their inventory on wheels which led to millions more trucks hitting the roads. The additional weight and rise in frequency of truck traffic led to myriad roadway and bridge failures.