Napier v. Atlantic Coast Line Railroad Co., , was a United States Supreme Court case in which the court held that the Boiler Inspection Act has so occupied the field of regulating locomotive equipment on interstate highways that state legislation requiring cab curtains and automatic firebox doors is precluded and must be addressed by the Interstate Commerce Commission.
The Supreme Court returned to the Boiler Inspection Act, now renamed the Locomotive Inspection Act, in Kurns v. Railroad Friction Products Corp., a case in which the court also held that field preemption applied.