A british TV light entertainment program of the end of the 20th century was "the wheel tappers and shunters club" which was a mythical working-mans club in the north of england. Like the music hall series "the good old days" did for pre-ww1 music-hall entertainment, it traded on a certain cliché of what a working mans club was like. They were pretty robust, a dive bar with cheap beer, darts and a pool table mostly. Groups like the Beatles would have toured them before they became famous. As did comedians, strippers, magicians, you-name-it. In my personal opinion, the entertainment was usually awful. But as a student, I drank in them because the beer was cheap (in York in the 80s)
The point being that "wheel tappers" really existed: they hit the wheels of wagons in the shunting yard with a hammer, to see if the tone of the cast iron wheel betrayed a crack developing. So I guess the MTA is just carrying on the tradition (ultrasound imaging and X Rays do this on aircraft)