Name one common radio protocol used in railroad communication.

The AAR (Association of American Railroads) protocol is a widely recognized and utilized standard within the railroad industry, specifically designed for communication among railroads and between railroads and other entities. This protocol establishes guidelines and procedures to ensure safe and efficient communication over radio channels used in railroad operations.

AAR protocol encompasses various aspects of communication, including rules for radio operation, frequency management, and communication etiquette specific to railway operations. This ensures that messages communicated over the radio are clear, concise, and understood by all parties involved, which is crucial for maintaining safety on the railroads.

While the other options reference important organizations or protocols relevant to transportation or communication, they do not specifically pertain to the unique requirements and standards for railroad communication. For instance, the FCC deals with regulating communications across all industries, NTSB focuses on transportation safety investigations, and RTCA is related to aeronautical communications, rather than railroads. Thus, the AAR protocol stands out as the correct answer for railroad communication practices.