Like Playing Subway memory

I've been living in NYC for almost a decade and have taken (and bitched about) the subway 1000's of times. Whether it's the service delays, the filthy platforms, the undesirables and of course the incomparable concept of not having Metrocard machines on the platforms.

But with all of these negatives and distractions I have yet to figure out when the N train will run on the express track and when it will run local and because the announcements are often so garbled it's like Christmas when it doesn't stop at Prince street and goes right onto Union Square.
For years I assumed the diamond or circle around the train symbol made a difference but that doesn't seem to always hold true, I guess like a subway time schedule the MTA will keep its customers guessing.
Sent from my Verizon Wireless BlackBerry