I grew up in New York and rode the subway alone at 9, and starting at around 11 it wasn't just to and from school but to go shopping or to run errands (I went through a June Cleaver-worship phase when I ran a lot of the household errands). I think that the independence that comes with growing up in a big city really shaped who I am today. Whenever I would meet kids from the suburbs I would kind of pity them, even for little things like having never tried Vietnamese food or never going to the opera---and they were in high school. My mum hated the opera, so I secured my seats and often went with a friend or alone. When I became obsessed with the idea of boarding school I got to hop on the Amtrak and hire drivers to go visit them alone since my mum didn't support the idea. At 18 I wasn't afraid to go abroad on vacation by myself and made many friends because of it. I never had a curfew (but then again I had disturbingly high morals). I didn't need to constantly check in and I think I had a greater sense of self at an earlier age. Things could have went wrong but they didn't. If kids were getting snatched off the train it would surely make headline news, being lucky is the rule and not the exception. I suppose that it depends on how comfortable parents are with the odds. The train cars are filled with people and the stations are only really empty during the workday and very late at night, someone would easily hear a child's cry, or so you'd hope. I always tied the really gruesome child crimes to places with more car culture, as in: "I have a puppy in my van, come see it" or "your mother told me to give you a ride home". Granted, I knew some kids who were limited to a certain block radius from their apartments, and others who were given hours for certain places, I was one of the few with carte blanche. Then again, I also never played in dirt, or learned how to ride a bike or do any of those risky suburban things that I heard so much about. So, after the rambling, it's all about your child and your comfort level.