The ZW is the most powerful of the old lionel transformers. Many people swore by them because they were more powerful than anything new that could be built according to Gov't regulations. In the last few years the manufactures in this scale have made replicas by side stepping the limits with more high tech do-dads. So the once highly valued transformers may be back on the market. If you want to keep prices low go for the older one if you can find one it will cost less and provide more power than anything else for the price.
The ZW was made to power and control two trains at a time. It has whistle and horn buttons. If you want to control another I advise getting another transformer. Actually I also advise the Greenburg toy train wiring guides as well. There are three and they demonstate different ways to wire you layout to do block control so two trains can be separately contolled on the same track or automatic control, fun stuff.
These are all older electronics in the transformer that are pretty high powered so be careful. The first thing to do is look at the cord if it is old of frayed do not use it.The best place to get one would be a store or train show that can test it and demonstate it in front of you. Have the one you are buying tested.
Good luck with your project.