I am not a terribly big fan of Agatha Christie, but "Mystery of the Blue Train" is one of her novels that I did in fact like and was therefore thrilled to hear that they've made this TV adaptation as part of the "Poirot" series with David Suchet.
Unfortunately, it was not to be... it started on the wrong foot immediately with the way it was directed--was there no one around to rein the director in? Everything seems shot in darkness, through smoke, half-obscured, half-reflected in mirrors, etc. It's often difficult to even see the faces of actors because of this.
Of course, that would probably be bearable if the screenplay adaptation wasn't so bad. It takes amazing liberties with the novel (some of which have been already mentioned in other comments) that I couldn't believe. The storyline doesn't seem to make any sense any more, and the dynamic between some of the characters has been completely changed.
In conclusion, do not go anywhere near this movie if you've liked the novel at all.