What constitutes an operative communicating signal appliance between the cars of a passenger train and engine?

The correct answer highlights that an operative communicating signal appliance between the cars of a passenger train and engine includes radio, electrical buzzers, and PA systems. These devices play a crucial role in ensuring effective communication between the train crew and the passengers.

Radios are widely used for two-way communication, allowing the crew to relay important information, safety announcements, or instructions swiftly and clearly. Electrical buzzers are often used to signal specific actions or alerts, ensuring that the communication is timely and can be acted upon immediately. A Public Address (PA) system is essential for disseminating important announcements to passengers, providing them with crucial information during their journey, including safety instructions or updates about the train's status.

Overall, these tools not only enhance communication efficiency but also contribute to the safety and comfort of passengers onboard, making them vital components of train operations.