I managed to finish this more quickly than usual, though a number of clues seemed baffling for some time. FOI & COD 11a, with 13d a runner up for COD. 18a was one of the clues I marked as being poor clues, so it was interesting to see that Geeker has it as a tie for COD; à chacun son goût, as they say in French. (And yes, I know that isn't the usual form
in English.) 6a was another of my poor clues, and I see that I am by no means alone there.
Several specifically American terms, some marked as such but not others. Maybe they have been imported into NZ English. As for the "nonsense from America", it is very much an Americanism to me, but Americanisms gradually creep into British English, and I don’t know whether British people half a century younger than me would regard it as American or not.
"Eccentricity" and "random" were both familiar to me as a mathematician, but I would think a bit obscure for most people. As for the "American railway", even my wife, who is American, wasn't acquainted with the expression. It occurs to me that you can read the clue for 1d as either "Dismisses (British neurologist)" or "(Dismisses British) neurologist", and it works either way. I wonder whether Pangakupu intended that double reading.