What is a condition for using the radio for emergency calls?

Using the radio for emergency calls is governed by specific protocols to ensure effective communication during critical situations. A crucial condition is that the emergency call should be repeated three times. This practice ensures that the message is clear and that it can be understood, even if there are disruptions or noise on the communication channel. Repeating the call helps confirm that the vital information is received and acknowledged by the appropriate parties, as clarity is paramount in emergencies where immediate actions may be necessary.

The other options present conditions that do not specifically address the appropriateness or effectiveness of the radio communication in emergencies. For instance, the presence of the radio on the train or issues related to channel congestion may impact communication but do not define the protocol for making emergency calls. Similarly, while the usage of radios is typically restricted to authorized personnel for routine operations, the emergency protocol focuses on the clarity and acknowledgment of the emergency call itself rather than its user or equipment specifics.