What should be checked if the whistle is not sounding even after the lever has been depressed?

In this scenario, inspecting the whistle isolating cock is crucial because it directly affects the flow of air or liquid necessary for the whistle to function. The isolating cock might be closed or malfunctioning, preventing the necessary medium from reaching the whistle mechanism. If the lever is pressed, yet there is still no sound, it's essential to check this component first to ensure that it hasn't been inadvertently shut off, as this would directly inhibit the whistle's ability to produce sound.

The other aspects, while potentially relevant, do not address the immediate issue as directly. For instance, while the power supply is important, if the lever is being depressed and the system is powered, checking the isolating cock becomes more pertinent. Similarly, although checking fuses is a good troubleshooting step, it may not directly relate to a mechanical failure in the whistle isolating cock, which can render the system inoperable regardless of the electrical integrity. Re-installation of the horn unit could address a separate issue, but if the isolating cock is improperly set, mechanical sound generation will not occur regardless of the horn unit condition. Thus, ensuring the isolating cock is operational is the most logical first step in troubleshooting this specific problem.