I'm a bit confused here.
I recently went on a trip to NYC. I took a 7-day Metro Pass (no express), since I was there for 5 days.
It seems that on the third day, I took an express train on accident. I thought I was screwed or something, but it seems nobody bothered checking my MetroCard, and on the way back nothing eventful happened.
So I'm curious. If most of the train lines don't even bother checking if you have express or a regular farecard, what would be the point of getting a MetroCard with Express? It seems nobody bothers checking people's farecards or anything, and though the thought of taking the express line from that day forth was tempting, I decided against it, and just took the local trains.