The real issue here is the fact that the whistle circuit was designed when the whistle tender consumed considerable power for it's AC whistle motor. The old PW transformers require significant current to develop enough DC offset to trigger the horn function, the modern locomotives and tenders don't provide enough load as a rule.
So, do you agree with Vernon's suggested remediation measures, or is there something else needed?
Background: I've been wrestling with this issue for several years now, ever since I upgraded to a postwar ZW (from several smaller PW transformers, one per loop, that I previously used). I was surprised to find the built-in whistle/bell switch did not activate the whistle tender I had at the time, even though direct DC (using an HO transformer) *would* activate and power the tender. I even picked up a couple of the add-on Lionel sound activation buttons to try, and dropped them (one at a time) in the circuit, using one of the smaller transformers for power, to no avail (and yes, I tried reversing the button's wires to reverse the DC polarity). I ended up creating an isolated track section on the layout, and used the DC throttle output from an HO transformer to activate the whistle tender when desired.
Fast forward to a month or two ago, when I bought a vintage Williams GG-1 with the True Blast sound system. I had to create a new loop on the test layout to accommodate the massive GG-1, which I powered with one of the small transformers. After boning up on the True Blast system, I resurrected the Lionel sound activation buttons and dropped them (back to back) into the power feed. In short, they sorta-kinda work, generally activating the horn/bell sounds but sometimes after a delay and sometimes not at all. At least judged by the immediate hesitation in the engine speed, the button's electrical offset seems to be as expected, but the GG-1's True Blast module seems to have some trouble interpreting the output of the buttons.
I have more fiddling and testing to do (trying the whistle tender on the GG-1 loop, and the GG-1 on a loop powered by the ZW), but before I do, any other suggestions on what to try to get things running/activating properly before I dive in (again!)? Thanks! 
