Anybody can perform a basic circuit breaker test by putting the ZW on a switched outlet (off). Set one pair of terminals to max voltage, and put a heavy gauge wire across the terminals.
Turn the power on and time how long it takes for the breaker to trip.
Compare your results with the tables in the test bench instructions that can be found in the Lionel service manual. (use the tables to also determine how long to wait before interrupting the test)
As far as keeping layouts safe: that is not what the internal breaker in a large transformer is supposed to do. It is there to protect the transformer itself. Use external circuit protection to protect your layout wiring and trains. Don't put the external breaker on the common leg. Put one breaker on each "Hot" terminal that is in use: "A", "B", "C", and "D". And if your trains have electronics, use TVS devices.
When servicing a Lionel transformer, don't forget to:
Check the cord
Check the rollers
Check for A.C. leakage
Make certain the whistle control(s) operate smoothly and do not hang up. *
I guess we all have our own standards. I would not use or sell a transformer that wasn't 100 percent in proper operating condition, as sold by the manufacturer, including the breaker.
* - had a ZW on my bench that had the whistle control hang up due to a broken centering spring. It created a short that was not protected by the internal breaker. The owner reported that lots of smoke was generated, and I could see evidence of parts like one of the the roller arms being overheated.