That's a little vague, I'm afraid. Is it a *A RTW, such as a CRWSTARx? If so, then, yes, you can easily take the train (i.e., surface) instead of flying those two segments. (Don't you have a copy of the Rules?!? You should have.) BUT you need change your ticket beforehand (routing change / reissue = $125) or, as others have noted, you will have trouble with the subsequent flights.

cheers,

Henry