The United States' largest railroad union, SMART, said on Monday it would back Union Pacific's proposed $85 billion merger with Norfolk Southern after securing job protection guarantees for its members.

The sector's largest-ever buyout, aimed at creating the first transcontinental railroad in the U.S., has won White House backing as Union Pacific and Norfolk Southern prepare to file a formal merger application with the Surface Transportation Board.

The transportation division of SMART, the International Association of Sheet Metal, Air, Rail and Transportation Workers, had previously said it intends to oppose the transcontinental merger citing concerns about how it will affect U.S. workers and infrastructure.

"SMART-TD members working in train and yardmaster service will have job protection for the length of their careers following the transaction," the union said.

"Union Pacific has committed that these employees will not face involuntary furloughs as a result of the merger."

Norfolk Southern and Union Pacific had previously said they intend to preserve union jobs and aim to be the safest railroad in the country.

SMART represents 230,000 members across several sectors including transportation, construction, and manufacturing.