Is it appropriate to change a baby's diaper on a subway or other form of public transportation?

I take public transit quite often, but today I saw something that I had never seen before. A woman stretched her toddler daughter(?) out on her back on the seat of the subway and then proceeded to change her diaper. (I don’t think she put a mat or blanket down first.) Just curious to find out what people out there think about this sort of thing.