My understanding is that the Z-4000 can put out more than 18VAC. I never trusted it after I read that on this forum. Ultimately I got rid of it and replaced it with Lionel 180W bricks. But I don't run any conventional so it was an easy decision. Still, an awful lot of folks are running the Z-4000 with no issues. Did MTH ever publish any service bulletins for that transformer?
More generally, the recommendation above about buying a UPS to isolate your gear from your power line is a good one. Just make sure the UPS you get is in fact one of the kind that isolate the incoming power--not all of them do that. Failing that, a good line filter is the right choice. Truthfully, everything that is sensitive in your house should be plugged into a line filter--audio gear, flat screen TVs, DCS trains--all of it. I found a well-documented 60A line filter and my entire layout is running through it with no issues. But I'm in the city and we have pretty stable power here.
Over the years, the power companies have been steadily increasing our line voltage to get more power out of the same wiring. 25 years ago, house power was 117V. 10 years before that it was 110V. Today it is usually a whopping 127V. Never assume that the power they are selling you is clean and well-regulated. Because you know what happens when you assume...