It occurred to me years ago that time travelers could travel within a slower than light interstellar spaceship, even a Heinleinian "generation ship" with many generations born and dying in flight. A time traveler whose existence was unknown to the crew but who appeared briefly now and then might be regarded as a ghost, hence a possible title: The Haunted Spaceship.
Another implication of this notion is that a time traveler entering the ship on Earth, traveling forward along the ship's world line to emerge at its destination, then returning along the ship's world line back to Earth would be able to report on the outcome of the mission before its departure.
Poul Anderson seems to have hit on the same idea in the concluding chapter of There Will Be Time (New York, 1973) but develops it slightly differently. He imagines not a generation ship crew with invisible time travelers moving among them but simply a ship crewed by time travelers for whom the journey will take only hours or minutes even if the ship itself takes centuries to cross an interstellar distance and he does not mention the obvious implication that the crew would be able to report back to Earth before their departure - although perhaps they would prefer to preserve the freedom of not knowing, a point which is mentioned in the novel.
Finally, a point that I could not have thought of, "...a mathematical equivalence between traveling into the past and flying faster than light..." means that the fact of time travel might enable physicists to develop an FTL drive. (p. 174)
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