(Reuters) - U.S. President Donald Trump signed an executive order on Tuesday to move forward with construction of the Keystone XL oil pipeline, rolling back key Obama administration environmental actions in favor of expanding energy infrastructure.   In 2015, former President Barack Obama rejected the cross-border crude oil line, seven years after it was first proposed, citing environmental concerns and saying it would not make a meaningful long-term contribution to the U.S. economy. Below are key facts and issues surrounding the project. The 1,179-mile (1,900-km) Keystone XL pipeline would move 830,000 barrels per day of oil sands crude from Hardisty, Alberta, across the U.S. border to Steele City, Nebraska, where it would connect with a previously approved line. The $8 billion project is backed by TransCanada, Canada’s No. 2 pipeline company. It requires a Presidential Certificate to allow the line to cross the border, which TransCanada first applied for in 2008.  Refiners on the Gulf Coast want the line to supply them with  heavy oil and supplant uncertain supplies from Mexico and Venezuela. Canadian oil sands producers want to tap the world’s largest refining market for their oil and boost the price they get for their crude.  Environmentalists have made pipelines the proxy for their battle against exploiting Alberta’s oil sands, saying production techniques there emit massive amounts of carbon dioxide, one of the greenhouse gases blamed for global warming. Obama expressed skepticism of TransCanada’s claims about the number of jobs the project would create and said he was concerned that oil extraction from Canada’s oil sands was “extraordinarily dirty”. TransCanada called the rejection “a symbolic gesture” aimed at burnishing the Obama administration’s image on climate change leadership.  Opposition in Nebraska from environmentalists and some landowners concerned about oil spills had been among several major hurdles facing the Keystone XL project. The line's route through Nebraska has been the subject of a court case in the state over whether former Governor Dave Heineman was entitled to approve the route. (reut.rs/2jbx05V) A Nebraska Supreme Court decision in 2015 ruled in support of the pipeline, but a number of Nebraskan landowners filed suits against TransCanada alleging the project violated the state's constitution. (reut.rs/2kpZzh1) In a separate legal action, TransCanada has been suing the United States in U.S. federal court, seeking to reverse the Obama administration’s rejection of the pipeline.  
Factbox: Trump gives nod to Keystone XL oil pipeline