Answer: A
Correct answer: No improvement
The sentence needs no improvement. When two past actions are described, the earlier takes the past perfect and the later takes the simple past. The train left first, so had left is correctly paired with reached. Putting both verbs in the past perfect - had left and had reached - would destroy the sequence, since the whole purpose of the past perfect is to mark one action as prior to another. If both are equally prior, neither is. Have reached is the present perfect and cannot be used alongside a past perfect describing the same episode. The same sequence governs hardly had he arrived when, no sooner had she spoken than, by the time we arrived, and reported speech, where a simple past becomes past perfect after a past reporting verb. The conjunctions before and after sometimes make the past perfect optional, since they establish the order themselves, but it is never wrong to keep it.