The recurring "issue" with these meters when used in O-gauge railroading is the isolation. As noted, for the ammeter an isolated power source (e.g., wall-wart) is required as it uses a shunt (resistor) to sense current rather than a CT "donut" which is inherently isolated. The "issue" is whether you then need 4 wall-warts or 4 independent/isolated power sources for your 4 meters.
Also, these ammeters are NOT true-RMS - that is, if they were I'm sure they'd be bragging about it! For voltage measurements from a Z-1000 brick, the waveform will be benign (sinusoidal) so you're OK without true-RMS metering. For current measurements, modern O-gauge engines have somewhat unusual current waveforms due to how most, if not all, command control electronics convert AC to DC. There have been OGR threads about this with the usual range of opinions about the validity, usefulness, whatever of current measurements if not true-RMS. No need to again stir up that hornet's nest so I'll leave it at that. 
