This is really obscure and vague, but any help at all is appreciated.
It's, I think, a young adult/teen novel, about a boy (?). There's some mystery about the boy's father and another dimension maybe? That's accessed through a subway terminal. I think it's set in NYC and that the boy lives with his mother in an apartment. I think there's something about the kid's birthday that triggers an odd series of events. I only flipped through it at my friends house when I was a lot younger, so I don't know all the details. Like I said, any help would be appreciated. Thanks!

I just recently read this book! It's part of a series of about 5 books - each is about a different person and situation but they're all kind of tied together. I have each book but unfortunately it's near 1 AM and I can't stay up any longer - I will look for them over the next 2 days and get back to you. I really liked the books (even though I am not a young adult, today's young adult books are so well written that they're really for everybody).

I found it! It's the first in the series "The Watchers" - it's called "Last Stop"! I read the first 4 books, they're really good.

Thank you so much! It was driving me crazy. I only read the first couple of pages, so I thought I was never going to find it!