In the case of United States v. Florida East Coast Railway (410 U.S. 224 (1973)), the Interstate Commerce Commission (ICC) was having a problem because railroad companies were just borrowing each other's freight cars instead of building new ones, leading to a shortage. They promulgated a rule imposing a fee for borrowing a freight car in order to encourage railroads to build their own freight cars. The railroad companies sued to overturn this rule. The ICC had used an informal rulemaking as defined in APA §553, but the railroads wanted a formal rulemaking as defined in APA §§556-557. The ICC's Enabling Act only said that a 'hearing' was required. The US Supreme Court found that a formal hearing is only required when the Enabling Act requires that the rule be made "on the record after opportunity for an agency hearing." If the Enabling Act just uses the term 'a hearing' (like it did in this case), then an informal rulemaking as defined by APA §553 is perfectly acceptable. The Court was very clear that courts cannot impose stricter rulemaking procedures on Agencies beyond what is set out in APA §553 and the Agency's Enabling Act. A formal rulemaking is a much more time-intensive process than an informal rulemaking. Formal rulemakings require oral hearings and are much more like full trials. Informal rulemakings only require a comment period for people to write in with their thoughts about the proposed new rule. There are also hybrid rulemakings, which are somewhere in between. These are not defined in the APA. See Vermont Yankee Nuclear Power Corp. v. Natural Resources Defense Council (435 U.S. 519 (1978)).