It doesn't need to be a hard distinction. I'm sure everyone can see the difference of a building with multiple platforms and usually shops and other things targetting non-locally-resident travellers and your average metro stop with has two platforms (one for each direction) and not much more than an entrance and exit. Where exactly you draw the line doesn't really matter.
That. That kind of distinction isn't universal. Some places just have a fused metro-plus-train system, in which you can slip in a $1 metro ticket, ride along changing metro trains for over a day across >1500km/1000mi, and pay outstanding fee at the exit, like on Japanese JR networks and metro trains interoperating with it.
I'm not saying which modes of thinking are more rational or superior or anything of that sort, just that the seemingly instinctive distinction you have described is not necessarily so to everybody.
