“You think someone on the train must have killed her, but that need not be so at all. What is to stop anyone swinging themselves on to the train when it stopped at Lyons? They could go straight to her compartment, strangle her, and take the rubies and drop off the train again without anyone being the wiser. She may have been actually killed while the train was in Lyons station. Then she would have been alive when Derek went in, and dead when the other person found her.”

Poirot leant back in his chair. He drew a deep breath. He looked across at the girl and nodded his head three times, then he heaved a sigh.

“Mademoiselle,” he said, “what you have said there is very just⁠—very true. I was struggling in the darkness, and you have shown me a light. There was a point that puzzled me and you have made it plain.”

He got up.

“And Derek?” said Lenox.