I just bought a used MTH Z-1000 that looks to be in great condition. I don't have a layout yet so I can't test it with a train, but I fired it up and checked it with my multimeter. I measure these AC voltages:
Brick 14V accessory output: 14V
Brick cable output that goes to Z-controller: 19.6V
Z-controller output with throttle set to full, no buttons pressed: 16.8V
Z-controller output with throttle set to full, pressing a horn/bell/whistle button: ~18V
Now, my multimeter is not the "true RMS" type, so I don't expect it to be accurate with a chopped sine wave and I have no idea how it handles the DC offset when buttons are pressed. But I was a little surprised that the max measurement without sound buttons was less than 17V (nominal voltage is 18V). On the other hand I have never tested the output voltage of another transformer and I have nothing to compare it to. Do my measurements seem reasonable for a healthy Z-1000?