Here's a thread that you need to read . Shows original breakers as well as new breakers that forum members are using. Also has a chart showing the amps output of each Lionel PW transformer.
Here's what I finally settled on for my ZW, a 10-amp breaker. Trips very quickly when track shorts. Plus you can't beat the price. I also bought four 5-amp breakers, one for each ZW output going to the tracks.