"Doubtless they had their reasons."
Lenox looked at him sharply.
"Do you know the reason?"
"I think so—yes."
Lenox sat still turning things over in her mind. Poirot watched her in silence. At last she looked up. A soft colour had come into her cheeks and her eyes were shining.
"You think some one on the train must have killed her, but that need not be so at all. What is to stop any one 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 any one 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 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.
"Who knows?" said Poirot, with a shrug of his shoulders. "But I will tell you this, Mademoiselle. I am not satisfied; no, I, Hercule Poirot, am not yet satisfied. It may be that this very night I shall learn something more. At least, I go to try."
"You are meeting some one?"
