Complex sentence: Leo returns to Moscow to investiagte the murder of another child, then returns to Raisa on a train, who has been visited by Leo's former rival, who tries to persuade her to leave Leo and ride a train with him to Moscow.
Split into simpler sentences: Leo returns to Moscow to investiagte the murder of another child, then returns to Raisa on a train. In the meantime, Leo's former rival has visited Raisa to persuade her to leave Leo and go with him to Moscow.