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By the time they reach the station the train would have/ will have left.
Since it's a future action, we should be using 'will have'.
But since 'would' can also be used as a modal verb to indicate a conditional mood, 'would have' also sounds good to me, as the one who is saying cannot be completely sure of the time of the train's departure.
Please clarify.
If it's in future tense, I would always use will have, even with a conditional context:
By the time they reach the station the train will have left, unless it is delayed
Would have only really makes sense if you're talking about a hypothetical past situation:
If they hadn't hurried, by the time they reached the station the train would have left.
(implying that they have already reached the station and the train hadn't left yet)
