The voltmeter I bought mentions "DC or RMS AC" in its list of input measurements, along with markings on its dial indicating voltage as "V~" (which I assume is AC) and "V" with a solid line over a dashed one. I was under the impression that it was accounting for the Z1000's output, but then again, I hadn't tested the cars on a Z4000 at this point.
I took the car to the test track and hooked it to the unused handle on my Z4000. The car lit up at 9.9 volts, I'm not sure at what point it actually starts at since the Z4000 didn't seem to want to output anything between 9.9 and 0.1 with just the single car (does the '4000 need a real load on it to output anything less? I've never used it in conventional). I've amended the post to indicate a Z4000's results and revised the Z1000's figure to a position of the controller's rotation.
I still think a passenger train heavy enough (like the Auto Train) to benefit from LED lighting would find it a challenge to get moving without the cars lighting up. If you're running 2-3 cars behind a Beep/f or a starter-set truck-mounted can-motor unit like the Lionel GP38, RS-3 or K-Line MP15/S2, then I guess one could have have issues keeping them lit at low speeds.
