I think you'll have to have both the whistle and bell. I made a whisle/bell box from junkbox parts, maybe you can go that route. The pushbuttons were just things I had laying around, same with the diodes. As you can see, there are two strings of diodes, and they are always in the circuit. When you push one of the buttons, it shorts out all but one of the diodes in one direction. This imparts a DC offset on the track and signals either the whistle or bell, depending on the polarity of the DC offset.
