Also you may consider a couple of N95 or better dust masks,
I had one in my kit when I was commuting by rail; I don't any more. I'm going to reconsider that decision.
pen sized high voltage detector (especially if you are in the city with a subway).
I don't think there's any way to safely test for train voltage with gear that isn't at least a yard long.
Agreed, especially in a city you don't know well. Keep in mind that most of the time you don't want to look like a tourist.