This subway mystery has gone off the rails.

MTA trains have been baffling riders by running with line numbers that have been defunct for decades – or never even used in the first place. 4

Photos on social media show people on trains advertised as being defunct 9 and 13 lines or — even more mysteriously — 10, 11 and 12 ones, which have never been NYC subway lines.

And while the MTA confirms that the mystery trains’ manual “roll signs” have been altered, it has no idea who is to blame — whether it’s a series of accidents by MTA workers , or pranksters having fun, with some even posting how-to videos online.

“Those are regular trains with test signage that may have been temporarily displayed incorrectly,” an MTA rep confirmed to The Post, saying that crews are supposed to check sig

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