Answer: because the train will arrive any minute.
Step-by-step explanation: An adverb is a word that modifies a verb, an adjective or another adverb in a sentence. An adverb clause is a group of words that function as an adverb in a sentence, it should contain a subject an a verb in order to be complete, and it starts with a subordinating conjunction (if, after, because, etc). In the given sentence, the adverb clause is "because the train will arrive any minute" because it has a subject (the train), a verb (will arrive), and it starts with the subordinating conjunction "because".