I've started to experiment with sound for AF trains. Here's my first attempt at replicating an Air Chime Whistle Generator with solid state components. The original ones used electro-mechanical vibrators to switch the track voltage between the voltage at the Variable output terminal and the 15 V accessory terminal. The circuit below just turns on and off the track voltage from the variablle output terminal at a frequency set by the variable resistor. As a result, this circuit, unlike the original Air Chime Whistle Generator, does not work without at least slightly turning on the track power. The optional resistor and diode will provide a capability to make a sound when the train is stoped, but it doesn't sound that great (Not that the Air Chime Whistle sounds very good in the first place). Note that one of the side effects of the original Air Chime Whistel Generator was that some engines could creep forward with the control knob turned off and the Air Chime Whistle Generator Activated. This won't happen with this emulation
I breadboarded the schematic linked below, but have not created a permanent circuit. Note that it is designed such that one could use the base for an Air Chime Whistle Generator and create a new "can" containing the circuit.
Air Chime Whistle Generator Emulator Schematic