USE OF SIGNALS 27. A signal imperfectly displayed, or the absence of a signal at a place where a signal is usually shown, must be regarded as the most restrictive indication that can be given by that signal. A signal unexpectedly found out of order or imperfectly displayed, or the absence of a signal at a place where a signal is usually shown, must be promptly reported to the Superintendent from the next point of communication at which the train stops. Engine and train crews using a switch where the switch signal is imperfectly displayed, or where signal is missing on a switch normally equipped with a signal, must, if practicable, correct or replace the signal. 28. A yellow and green signal by day, yellow light at night, will be used at stations to stop a train where flag and conditional stops are indicated on its schedule. 29. When a signal, except a fixed signal, is given to stop a train or engine, it must, unless otherwise provided, be acknowledged as prescribed by Rule 14(g), (h) or (r). 30. The engine or control car whistle must be sounded when passing trackman's portable whistle sign; also when view is obstructed by snow, weather and other conditions, approaching interlockings, stations, yards or other points where men may be at work. The engine or control car whistle must be sounded or bell rung while approaching and passing public crossings at grade; while passing through tunnels; also as a warning to persons observed or known to be on or near the track. All trains must sound engine or control car whistle as prescribed by Rule 14(m) when operating on a track adjacent to a station platform during hours of daylight; or when engineman observes a person or persons on or near station platform during hours of darkness. 32. The unnecessary use of either the whistle or the bell is prohibited. 33. Watchmen stationed at public crossings at grade must use stop signals when necessary to stop trains, engines or cars. They will use prescribed signals to stop highway traffic. 34. All members of engine and train crews must, when practicable, communicate to each other by its name the indication of each signal affecting the movement of their train or engine.