Imperative or Interrogative?

Which is correct: "Guess who won?" or "Guess who won." Could this very sentence be both interrogative and imperative depending on the context? Here's the context: "The train barrelled down the track. A cow stood in its path. Guess who won?"

Please clarify for me.

growinup

Thanks CJ,

Would these imperatives require a question mark as the end punctuation?

Guess who won? (i.e. Imbedded interrogative - Can you tell me who you think won?)
Tell me who won? (i.e. Imbedded interrogative - Are you able to ...?)
etc., etc.

Is the '?' correct in this case?

growinup