In the short story The Girl in the Train, Betty Brighteyes is the accomplice of Mardenberg.
Betty Brighteyes does not appear in the story in person nor is there any description of her. However in the episode The Case of the Middle-aged Wife of The Agatha Christie Hour, an TV film adaptation of the story, the main protagonist George Rowland gets to meet Betty at the end of the show. She has been arrested by Inspector Jarrold. She looks like the maid at the hotel which Mardenberg and George stayed at. The part of the maid is played by Debbie Farrington.