I have adapted the sentence. I've changed the name and the means of transport. The original sentence, which is different from the one above, comes from a simplified version of Agatha Christie's Murder on the Orient Express (Pearson English Readers p. 2).
A "cool and efficient" voice is a voice which makes the speaker seem cool and efficient - the voice of someone who is cool and efficient. It's better, incidentally, not to adapt sentences, as you may inadvertently change something which makes a difference to the meaning.