The Mail Divisor Cases: Northern Pacific Railway Company v. United States, 1919, revolved around the issue of how to calculate compensation for railway companies transporting U.S. mail. The Postmaster General had been using a divisor (a number used in division) based on an average weight per cubic foot that was lower than what the railways believed it should be, effectively reducing their payment rates. The Northern Pacific Railway Company sued over this method of calculation and won in lower courts; however, upon reaching the Supreme Court, the decision was reversed. The Supreme Court ruled that while Congress had set specific rates for rail transportation of mail by weight and distance traveled, they did not specify how to determine volume when calculating weight - leaving it up to administrative discretion. Therefore, as long as there were rational grounds for his choice of divisor (which there were), then he acted within his authority and his actions could not be overturned by a court.

In the dissenting opinion for Northern Pacific Railway Company v. United States, also known as The Mail Divisor Cases of 1919, Justice Holmes argued that the majority's decision to allow Congress to set rates for mail transportation on railways was an overreach of legislative power. He contended that while it is within Congress' authority to regulate commerce and ensure fair pricing, this should not extend to dictating specific prices or rates without a clear basis in fact or reasonableness. Furthermore, he expressed concern about potential violations of due process rights if railroads were forced into service at unreasonable rates dictated by government fiat rather than market forces. In essence, his argument centered around maintaining a balance between necessary regulation and protection against arbitrary governmental control.