We generally don't think of them as one, but subway turnstiles are a piece of interactive technology. Their single interaction, the swipe of a Metrocard or pass, produces physical feedback in the form of the turnstile either letting the person through, or locking. It also displays text on a small LED display, and produces a beep - one beep if the swipe is valid, two if not. In Chris Crawford's terms (The Art of Interactive Design, 2002), the turnstile "speaks" to the person through physical, auditory, and textual means.
For our "Intro to Physical Computing" class, I observed people using the turnstiles. Due to the simplicity of the interaction, there is little variety in how people use the machine. They quickly swipe their card and move through, usually only taking a second or two. Some pause before starting, to fish out their cards. Almost nobody looks at the display, unless their swipe fails. Occasionally you will see someone watching for a successful swipe, or searching visually for a moment or two to figure out where the card reader is - presumably these users are tourists, a very different context of use from commuters.
The most common difficulty is swipe failures. The sound for a failed swipe is poorly chosen, because it is very easily confused for the sound of a successful swipe, especially since it begins identically. A double-beep is easy to miss when many turnstiles around you are beeping from people passing through. Most people I observed don't notice their swipe failed until they hit the turnstile and can't pass through. One woman did this three times in a row before stepping back and watching the screen as she swiped.
A differently pitched tone for the failed swipe would be a noticeable improvement. One could also place a light above each turnstile, and have it turn green or red depending on whether the swipe succeeded. The light should be placed prominently on the arch of the turnstile, where the person is about to attempt to walk through - people will have already moved past the card reader and would not notice a light placed there. However, given that most commuters move quickly through the turnstiles, barely aware of an interaction that has become reflexive to them, it is likely that physically locking the turnstile will remain as the most common way that the turnstile speaks to the user.