Small pop-up coffee stand in the front portion of the Romper Room (a bar at night).  Had an excellent cafe-au-lait and had zero wait for it (sure, it was about 7:15am, but if you're reading this and happen to be looking for a good coffee bar near Union Square without a line at 7:15a, well, Iron Horse is your place!). Barista was super friendly.  Will definitely go back!