In my opinion, even if certain words don't have a translation, there is always some way to communicate the same (or almost the same) concept in the other language (english in this case).
i think the only times ive ever felt restricted by english is when writing fiction, because sometimes adjectives in my native language that i would like because i like how they sound come up but of course i cant use them and their english equivalent is just not really the same... even if the meaning is.
Makes me want to just mix it all in a retarded Finnegans Wake way. In my head both languages are mixed up anyway, so.
I think the train thing might be because they are very specific lexical fields that you never really learn in classes or through normal conversation (real life or internet). That happens to me too, also with things like the parts of a car and types of fish or verbs about specific cooking techniques. I'd say I know it mostly, but not everything.
Once as a kid I got really really shocked because I realized I didn't remember the word for "fire hydrant" in my native language and I could only remember it in english. I searched it in google translate but it gave me another synonym that must be use in another region and i spent like ten minutes searching until i found it. I found it so baffling I don't think I'll ever forget it LOL.
Interesting thread, I hope it somehow picks up. There's no way to accurately gauge this but it would be interesting to know the percentage of non english speaker kiwis.