This button is design to tell the engineer if the grades are to steep and if the engineers are not awake. The device will also count down from any number (in seconds) depending on the locomotive's speed. If the alerter is not reset when the timer reaches zero, the locomotive and any attached locomotives would be thrown into the equivalent of "emergency" and the locomotive brakes would be applied immediately.
Because the air had not been connected to the rest of the train, the locomotive "independent" brakes would apply.
The train would be stopped, but not as quickly as if the rest of the brake pipes on the trailing cars were connected.
On steam locomotives they don't use these buttons because the grades are not too steep.
These buttons are only seen on diesel and electric locomotives.