From the first time I laid my eyes on the Brooklyn Bus Map, I haven't seen the purpose of justifying the existence of the B39. Not a temporary shuttle, but an official, permanent bus route that duplicates the line between just two consecutive stations doesn't make sense to me. If I'm in the vicinity of the Marcy Avenue Station seeking a destination in Manhattan why would I take the B39 if I'd still have to transfer to the train at Essex Street anyway?