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.