The pipeline, originally expected to cost about $8 billion, was to carry about 830,000 barrels a day of crude oil, virtually all of it coming 1,210 miles from the tar sands to Steele City, Neb. A southern leg from Cushing, Okla., a major storage hub, to the Gulf Coast was approved in 2012 and constructed. Most of the northern leg has also been completed, with some of the most important gaps in Nebraska.