Have you ever used a concession myki at one of those barriers at Melbourne Central and then had an authorized officer approach you asking for your proof of concession? They always seem to be able to pick out passengers using concession myki's with almost 100% accuracy.
There's an ingenious way that Metro Trains (current operator of the metropolitan suburban train system) alerts them. It's all to do with the design and one specific light on the fare gates.
On top of the right barrier, after you tap your myki, one of the 2 dome-shaped looking lights will flash every time someone with a concession (or non-full fare) myki taps on or off. You can try it next time! (*only if you are actually entitled to a concession fare...I am not promoting fare evasion here*). This alerts the authorized officers who stand outside the fare gates stopping and checking the concession entitlements of passengers as they pass through the fare gates.
On most other myki readers, (including on buses and trams) they will beep 2 times when a concession myki is touched on or off and only beep once when a full fare myki is touched on or off. However, I have never noticed an authorized officer use this knowledge to check concession passes of anyone who touches on or off (it's usually just random spot checks of everyone on the tram or bus).
Contrary to popular belief, the officers are not there to stop barrier jumpers, even though they can (and probably will if they see it happening...would highly not recommend doing this as this is both illegal and dangerous as you may injure yourself attempting this, thus reducing your ability to escape to safety). They are mainly there just to check concession passes!
On another note, this also occurs in Sydney where a light on top of the adjacent barrier will illuminate when a concession opal card or single use ticket is tapped on or off along with making a different tone, allowing ticket checks to occur more efficiently and the enforcement of concession entitlements.
I am not aware of this occurring in cities such as Perth, but this may just be because single use tickets there are paper and are verified in person with random checks on trains, buses and sometimes ferries. Also, the reason it may not be necessary in Perth might also be because concession smartcards (in Perth, they are called SmartRiders) are comparably difficult to obtain as they require proof of concession entitlement at the point of issue and many of them have personal details of the holder displayed on the back of the card.
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