Are "我在哪里可以买火车票" and "我哪里可以买火车票" both correct sentences? Are "我在哪里可以买火车票" and "我哪里可以买火车票" both correct sentences? If one isn't, why?
"我在哪里可以买到火车票?" is the correct way to ask where you can buy the ticket, but in oral language "我哪里可以买到火车票?" is used quite often.

Actually there are 2 ways how "我哪里可以买到火车票"  can be interpreted. The first one is a question asking where you can buy train tickets. The second one is a statement meaning there is no place where train tickets can be bought. In oral language people will know what you mean by the context, the tone of your voice and your expressions.