Negotiations at one point and time were set to take nearly 100,000 barrels/day of Bakken crude, which is nearly 10% of the nearly million barrels/day produced. Currently western ND sends out 10 trains/day to carry oil, which is fairly significant. A lot of those cars probably shouldn't be holding hazardous material, but it's going to be a significant investment (billions and billions to fix them). New pipelines are significantly safer than trains or old pipelines. They can be remotely and intelligently analyzed for pressure drops and pipe density to monitor any corrosion or leaks. In my opinion, talking about new pipelines and older pipelines is apples to oranges, and they are significantly safer than trains (I believe it's 3 significant Bakken oil carrying train crashes/derailments in the last 6 months).
Build the Sandpiper.