There was case like this in Boston about 15 years ago that came to light as they were switching away from tokens to Charlie Cards. It was an MBTA mechanic. IIRC he was caught because it looked strange that someone kept showing up with bags of tokens to exchange for cash at one of the stations.
Even with the new card system, someone figured out a scheme to steal:
https://www.universalhub.com/2011/mbta-revere-man-made-sold-...
And when I was a kid there was a big parking meter scandal in which most of the collectors were implicated after a resident saw two of them skimming. Loved this quote from the investigator:
''The meters in Boston are the oldest I've ever seen,'' Mr. Vitagliano said. ''In some cases, it is actually easier to take a quarter out of one of our meters than to put a quarter in. I mean that.''
https://www.nytimes.com/1982/07/08/us/boston-meter-collector...