Plot holes? (SPOILERS!!!)
I don't know if these are plot holes or just sloppy editing, but there sure are some unanswered questions at the end of this film.
1. As Holmes himself notes in the middle of the film, the mother of the first murdered man seems curiously unaffected by her son's death. This is clearly indicated in the film as a reason to consider her as a suspect and Inspector Lestrade even decides to interview her a second time due to it. At the end of the film, this is entirely unexplained (and we never hear what emerged from Lestrade's second interview).
2. An explanation is provided at the end of the film as to why Holmes suspected that the policemen who come on board are actually accomplices of the killer, but no explanation is ever given as to why Holmes has correctly deduced the identity of the killer (except possibly that the poison used would be familiar only to someone who has spent time in India, but as the "identity" of the killer during the film is a fabricated one, how would that be relevant?). False leads to both the aged professor and the train attendant are given, due to their interest in mathematics, but no real leads given as to why the criminal mastermind is the person who is revealed to be him at the end.
By the way, did anyone else think that the acting of the woman was incredibly wooden?