I don't believe I have used Farringdon before. Certainly not since Oyster was introduced. On arrival from Liverpool Street, I found a yellow Oyster card reader near the platform, which I used to touch out. Shortly after I came to the Turnmill Street exit gate line. The staff let me out and the system came up with the desired result, so no complaint or dispute. I am aware of the pink validators. What I don't understand is why there would be a yellow card reader near the platform of a station with gate lines. Anybody know why it is there ?