The problem with ZW and most or all older transformers is NOT necessarily the breaker, the rollers, or the cord. Many older transformers used what I would call "paper" for insulating the individual segments of the punching stack, and as this "paper" aged and deteriorated with heat the stack gets loose. That is why many old transformers "humm". That is 60 Hz 120 volt power vibrating the stack. A loose stack, in addition to reduced efficiency and the result, running hot, is harbinger of a potential future transformer failure. I am not aware that any "overhaul sites" for ZW's are restacking the transformer core. I have had a few ZW's, and they do not even have an Off-On switch. If you forget, even once, to unplug or disconnect a ZW after operating your trains, you are at risk.
If you value your trains, and possibly your house, get rid of all this old stuff and buy something with a modern, higher rated, and non degradable insulation system and UE protection.
My $0.02
