When, however, the train had completed its slow journey round the ceinture and reached the Gare de Lyon, Poirot became suddenly galvanised into activity. Van Aldin realised that part of his object in travelling by the train had been to attempt to reconstruct the crime. Poirot himself acted every part. He was in turn the maid, hurriedly shut into her own compartment, Mrs. Kettering, recognising her husband with surprise and a trace of anxiety, and Derek Kettering discovering that his wife was travelling on the train. He tested various possibilities, such as the best way for a person to conceal himself in the second compartment.

Then suddenly an idea seemed to strike him. He clutched at Van Aldin’s arm.

“ Mon Dieu , but that is something I have not thought of! We must break our journey in Paris. Quick, quick, let us alight at once.”