I can say that on May 8 a whole bunch of achievements unlocked at once, but it is still very sporadic. Since then, I have done some digging and have found that you can see what is pending to be unlocked in your “output_log.txt” file that gets generated each time Mini Metro launches.
For example, from mine:
Profile: Completing achievement shanghai2 remotely.
This means that, at some point, the “Shanglow? Shanghai!” achievement will unlock. It seems to only be possible at game launch, and is seemingly random. I can launch the game ten times and nothing will unlock, or launch it twice and something new might unlock each time.
When something has been unlocked, that log will read something like:
OriginContext: Recording achievement Earl Grey (43) as complete.
For what it’s worth, I have only seen one achievement unlock when it should, while playing, as I fulfilled the requirements. Every single other one is essentially queued and will unlock on some random future game start.
So, if you want something to unlock, first check the aforementioned log to see if anything is pending, then just open and close Mini Metro until it does.