These are the MetroCard turnstiles present in every station. These stainless steel turnstiles feature readouts that tell you how much value is left on your MetroCard as you swipe it through the reader slot (visible as a block on the top surface of the turnstiles in the right-hand photo). The curved bars on top carry electrical and data circuits, and are also supposed to thwart turnstile jumpers. They can be found in almost any quantity from two on up, and are usually positioned on the mezzazine level, unless it's a single-level station, in which case they border, and extend into, the platform area.
