Like the heads in a VCR, the ones in card readers can wear out. After all, they are reading cards at an extraordinary rate. The busiest turnstile in the subway system, turnstile No. 10 in the middle array by the escalators in the main entrance to the subway below Grand Central Terminal, reads a whopping 236,000 cards a month.

I thought that was a neat factoid. I can imagine New Yorkers saying to themselves, "I know that turnstile!"

The article is rich in trivia about heavy-duty magnetic card readers and the millions of people who (ab)use them.