I have assembled you here in this, the dining car, because I am now ready to reveal the identity of Colonel Rumsworthy's killer.
It was not — as many, including Detective Sergeant Mewler here, have suggested — the beautiful Penelope Jule, star of stage and screen: she was otherwise occupied in the Bundermans' compartment seducing Mr. Bunderman… and his wife. Nor could it have been the Rector as he is left-handed and therefore completely incompetent. Even the locomotive's coalman has been ruled out, as he would have left a far greater, sootier mess.
It was I. I am the killer.