Thomson v. Pacific Railroad was a landmark United States Supreme Court case that established the principle of federal preemption in the area of railroad regulation. The case involved a dispute between the Union Pacific Railroad Company and the Central Pacific Railroad Company over the construction of a bridge across the American River in California. The Union Pacific Railroad Company argued that the bridge was necessary for the efficient operation of its railroad, while the Central Pacific Railroad Company argued that the bridge would interfere with its own operations. The Supreme Court held that the federal government had the power to preempt state laws in the area of railroad regulation, and that the Union Pacific Railroad Company had the right to construct the bridge. The decision established the principle of federal preemption in the area of railroad regulation, which has been applied in numerous cases since.

In Thomson v. Pacific Railroad, the Supreme Court was asked to decide whether a railroad company had the right to take private land for public use without providing just compensation. The majority opinion held that Congress had not given railroads this power and thus they could not take private property without paying fair market value for it. Justice Field dissented from this decision, arguing that under the Fifth Amendment of the Constitution, which states that no person shall be deprived of life, liberty or property without due process of law nor denied equal protection under the laws; Congress has implied authority to grant such powers as are necessary and proper in order to carry out its constitutional duties. He argued further that since Congress had granted railroads certain privileges in order to promote transportation between different parts of the country, it would be reasonable for them also have been granted some form of eminent domain power over private lands needed by those companies in order fulfill their obligations. Therefore he concluded that taking land from an individual should only occur after payment is made at fair market value and with due process being followed throughout all proceedings related thereto.