Originally Posted by og fan:
GRJ. just curious, whats the reason the diode string it tapped into between diode 4&5 rather than a direct jump over ?
Because that's the way it works! 
The diode strings when you don't have any buttons pushed just drop the AC voltage, typically by about .6V for each diode. When you push one of the buttons, you short out four of the diodes and introduce a DC offset of around 2-2.5 volts. The polarity is dependent on which button is pushed, hence the plus or minus for whistle or bell.
If you shorted all the diodes out, you'd create plain DC on the track, that's not what you want, you have to leave at least one diode in the mix.
