Erie Railroad Co. v. Tompkins. In a landmark decision, the U.S. Supreme Court ruled in 1938 that, in cases between citizens of different states, federal courts must apply state common law, not federal "general common law." Under Pennsylvania common law, Harry Tompkins of Hughestown lost his case against the Erie Railroad, a New York State company. Tompkins had been struck by an unsecured door of a passing train and severely injured near this spot on July 27, 1934.