Union Pacific and Norfolk Southern are facing pressure from rival railroads BNSF and Canadian National, who told the Surface Transportation Board that the companies are withholding key documents needed to evaluate their proposed transcontinental merger, reported New York's FreightWaves.

In filings, BNSF and CN said the STB cannot properly assess the merger's impact on competition without more internal records. Both railroads asked the regulator to compel UP and NS to produce board presentations, adviser analyses and internal emails.

BNSF argued the review schedule should be paused until the documents are provided. It said the withheld material could show whether executives privately acknowledged risks such as reduced service, higher rates or failed integration.

CN took a narrower stance, saying UP and NS withheld Schedule 5.8 of their merger agreement. The document likely outlines how the companies assessed regulatory risk, potential divestitures and remedies. CN's legal chief Olivier Chouc said transparency is critical given the scale of the deal.

CN also claimed the omission leaves the 6,692-page merger application procedurally incomplete under STB rules. The regulator is expected to decide shortly whether the filing is sufficient or must be resubmitted.

Both BNSF and CN warned that UP and NS's public statements may not match internal discussions. CN added that the application lacked market analyses, traffic projections and competitive proposals. Canadian Pacific Kansas City has backed CN's motion.

Union Pacific said it stands by its application. Spokeswoman Kristen South said the company is confident in its merits and will continue working with the STB during the review process.