The cost inflicted by oil is measured in centuries. A year or two saved may not make a difference in terms of total lives lost:

http://www.telegraph.co.uk/earth/environment/globalwarming/1...

Moving oil through the pipeline is bad, but it is less bad than moving by rail in light of the probabilities of human loss (though the spillage is likely to be bigger when there is an accident). At least there is an potential up of rail being freed up to move something else safer - like people.