You mention bell. If you have a diode rated higher than a transformer amp output you can blow the whistle/bell with it strung from center to outer rail (hold with pliers, it will get hot, fast...the smaller amp the hotter....an 8a might work for a sec or two, but too much will cook it fast. This is for tests the whistle works at all)
The boards look more for wave shape than offset max is my guess (fast change). The electromechanical relay likes a heavy offset and doesn't care much about wave shape, it relies on the peaks more to do actual work.
The old whistles are on a 5v boost too. This is to make up for old whistle motor draw. New whistles use can motors or recorded sounds and don't need a boost to keep from slowing, so new transformers don't give a boost.
You can bypass the ZW boost or leave it when swappimg in a diode. (5v boosts can cause new locos to rocket ahead)
Building a box you can taylor boost as needed and include whistle and(or) bell
It just takes a handfull of diodes, and a button. Some extra switches for choice of wh/bell, and boost level (rotory or toggles) are optional.