I don't have any Postwar engines or Pulmor engines anymore. All of mine are modern can motors.
I did everything I mentioned in my last post, and ran the two largest engines and lighted caboose at full throttle, and I measured 5.1 volts on every section of my track.
So, . . is this bad, or good, or good enough?
That should be fine. If you didn't see any significant slowing of the engines or the lights on the caboose/engines dimming significantly (ie where you notice it), then that should be fine. If you are running modern can engines then 18 gauge wire is perfectly safe the way you are using it.