The revised $85bn filing says the tie-up would create 1,200 jobs by year three, save shippers $3.5bn and move 2.1 million truckloads to rail.
The Surface Transportation Board has 30 days to decide whether to accept the application after rejecting January's filing as incomplete, with a full review expected to take more than a year.
Rivals, shipper groups and some unions warn the deal could raise rates and weaken competition, while Union Pacific added concessions on St Louis terminal ownership and disclosed a $2.5bn breakup fee.
Could the merger of Union Pacific and Norfolk Southern trigger a wave of further railroad consolidation, reshaping America's entire freight and passenger rail landscape?
What safeguards might the STB impose to ensure this mega-merger doesn't lead to higher shipping costs or degraded service for shippers and consumers?