"Operates fine elsewhere" and "breaker never trips"; these statements mean the Z and loco are ok and your issue IS track based. It almost has to be.
Flip the meter to ohm (the upside down U symbol with feet) and put leads in the com. and ohm jacks. When you touch the leads together, you should read 0. or real close too it (some meters will never actually read 0 but are 'close enough'.
Now , no power hooked up, slightly pull track apart about an inch. Read from pin to hole and you should get a low reading or 0. Problem rail(s)will read higher than a good ones , or "bounce" as the test current pulses/ surges from trying to literally jump across a poor connection. (some digital meters will just "hunt" endlessly with no reading and some have peak/hold options...etc etc)
Now you know which rail has issues, so now move around the layout measuring just that rail at arms length 3-4 pieces at a time of it, till you find the bad area. Then single it out.
AND OR disconnect track near the power, run the engine railed on the power side till it stops. There is your gremlin, look hard.
Shine all connection tabs, etc. with a wire brush or contact cleaner, etc. They are likely clean looking, but that doesn't mean a whole lot with plated metal. It seems the oxidation is invisible or something on modern plated metal connections. (outside the train world too). Whatever has changed in the plating process doesn't seem like an improvent to me really.
Check pin fit too, look for possible shorts in underside wire, etc.etc
Assuming that has all been done over and over, and not in haste.... Move the tracks power connection a few feet. Does the problem move? Yes- Add a second power connection and it should "balance". No- replace that piece of track or run wire to jump it into the circuit.
....Your not running a bunch of lighted passenger cars too when you have issues are you?
