Correlative conjunctions are pairs of conjunctions that work together to connect words, phrases, or clauses. In this case, "neither...nor" is the correlative conjunction connecting "the bus" and "train."
Here's why it's not other types of conjunctions:
* Coordinating conjunctions (like "and", "but", "or") connect items of equal grammatical rank. While "nor" can be a coordinating conjunction, in this sentence, it's part of a pair.
* Subordinating conjunctions (like "because", "although", "since") introduce a dependent clause. There's no dependent clause in this sentence.
Let me know if you'd like more examples of correlative conjunctions!