Re: Painting pictures of graffiti train cars?
Have you taken pictures of Banksy graffiti, or Shepard Fairey graffiti - if so i would advise you to not put those in your book.
Having said that if I were you - i would go forward with the paintings. If you took a photo of a building with a coca-cola logo on it, then made a painting of it - is that copyright infringement?
Making a book, rather than a one-off painting further muddies the water.
There are a number of published books of graffiti on subway trains, etc.- I would contact the author and/or the publisher to get some help. And while I had the publisher on the phone I would pitch the book to him.