What action must be taken if the horn or whistle fails while en route?

When the horn or whistle fails while en route, the appropriate action to take is to notify the dispatcher immediately. This ensures that the railroad is aware of the situation and can take necessary actions to mitigate any potential safety risks. The dispatcher plays a crucial role in coordinating train movements and communications, so informing them of the failure allows them to provide guidance and ensure safe operations.

Prompt notification also helps in assessing the situation, as the dispatcher may need to adjust signals or manage other trains in the area to ensure safety while the train continues its journey without the required signaling capability. This proactive approach is essential in maintaining safety standards on the railroad.

The other options do not address the safety protocols necessary in the event of a horn or whistle failure and may lead to dangerous situations if not handled properly. Increasing speed could compromise safety, while continuing without making any signals would not adhere to the required signaling protocols. Checking for additional signaling devices is a relevant action, but it should be preceded by notifying the dispatcher about the malfunction.