A. There is no reason why your Lionel ZWs and Flyer “big ones” can’t be used to run AC-powered trains of any vintage – new or old.
The only time you might not get your full money’s worth is during operation of locomotives equipped with Lionel’s TMCC or MTH’s DCS command-control systems. While those locomotives and their sound systems are designed to operate using conventional transformers like your ZW, digital controllers are needed to activate additional features like special sound effects and coupler activation. Also, you won’t be able to take advantage of independent control of multiple engines on the same track without a digital controller.
I have had the same problem. I recently acquired a beautiful Lionel NH Alco 420. It does run on conventional power but not great and no whistle or bell. Not wanting to give up, I experimented with an 80 watt modern Lionel transformer and got great results. Apparently there is a difference between old conventional and new conventional power. I have since wired the new transformer to my layout along with the old ZW.
I will make a note here. I have noticed that my old transformers do not work well or not work at all with newer electronic whistle tenders. On the flip side, the new transformers do not seem to work as well with the prewar whistle tenders.
Has anyone else noticed this anomaly?
Older transformers I had this issue with are RW, 1033, KW, ZW….newer transformers were all made by MTH. I have purchased diodes to put into the older transformers to see if this resolves the issue with newer tenders.