Observation 1: A good number of NYC commuters are "book readers". In Singapore, you can hardly find commuters who still read actual physical books on the MRT. In fact, the number of e-book readers is also quite small.
Observation 2: NYC commuters are not engrossed with their phone or electronic device. The "engagement" is usually limited to listening to music from the iPod / phone. Singapore commuters spend much more time on their phones or tablets - either browsing Facebook, playing games, and/or watching videos - which bring me to the next point.
Observation 3: No one watches downloaded videos on their phone in NYC. In Singapore, a large number of commuters have HK, Korean, US dramas loaded on their phone. NYC residents must be fulfilling their TV time at home I guess.
Observation 4: The New York subways are loud because of the old infrastructure it is running on. However the noise helps to eliminate other common noises we hear while commuting in Singapore - such as people's conversations on the phone & each other, as well as the irritating ear phones that unnecessarily project (usually) techno music to everyone else in the carriage.
Observation 5: There are less "sleepers" in the NYC subways compared to in the Singapore MRT. Perhaps their commute time is shorter? Or perhaps it's in their nature to keep alert for safety reasons?
