A information security metrics piece in our local newspaper caught my recently. To be honest, it didn't actually use the word "metric" as such, nor "information security" for that matter, but that's what it was.
Like many others, the train company in Wellington NZ has a problem with fare dodgers. Some bright spark in their internal audit team, I guess, realized that comparing the number of people who use individual trains with the number of tickets sold would give them a huge clue about which trains and stations should be at the top of the ticket inspectors' hit list.
Counting passengers would be a tedious and error-prone job for a person, but an infra-red beam across the carriage doors would do nicely - particularly as the hardware may well already be installed as part of the door control and safety system.
The automated count will inevitably have errors (e.g. passengers who alight at the wrong stations then rejoin the same train), but provided the counting system is correctly configured and calibrated, the errors should be within known bounds and good enough for the purpose. Likewise the number of tickets will have genuine errors, for example passengers with season tickets who neglect to swipe them. The absolute number of passengers traveling is less important than the relative numbers of passengers and tickets: the further apart they are, the more likely something untoward is going on.
I imagine the statistics will be presented graphically, showing a breakdown of the number of passengers and corresponding number of tickets for various journeys. Those with the greatest discrepancies would naturally be targeted by the inspectors.
I imagine also the graphs will have a few empty slots where the ancient rolling stock breaks down - which hints at another important metric for the railway: service reliability. Conceivably a greater number of passengers will be prepared to pay their way if the trains were modern, comfortable, fast and reliable. But perhaps I'm being overly cynical. Fare-dodgers aren't helping since their payments would help fund the upgrades needed.