Robert McBride (defendant) worked as a locomotive engineer for CSX Transportation, Inc. (CSX) (defendant), a company that operated an interstate system of railroads. After seriously injuring his hand while operating a train, McBride underwent two surgeries and extensive physical therapy, yet he never regained full use of the hand. McBride commenced a Federal Employers Liability Act (FELA) negligence action against CSX in U.S. District Court alleging that the equipment he operated was unsafe and that CSX had failed to properly train him to operate the equipment. One of the jury instructions requested by CSX would have required McBride to show that CSX’s negligence was a proximate cause of his injury. Another instruction would have defined “proximate cause” to mean “any cause which, in natural or probable sequence, produced the injury complained of.” The district court refused to give the requested instructions. A jury found for McBride and CSX appealed. The court of appeals affirmed the jury’s verdict and the U.S. Supreme Court granted certiorari to review.