f this were to be a variant on the famous Agatha Christie train story, then "Hercule Poirot" may have ended up having an entire train's worth of murders to investigate. Rarely, can any one journey involve so many miscreants as this one from Paris to Rome. Geographically, it isn't that far - but when a man goes aboard intent on acquiring a recently stolen van Dyke painting - the folks who originally pinched it are also on the hunt - it takes on dangerously risky proportions. Conrad Veidt is particularly effective, as is Sir Cedric Hardwicke as the publicity-seeking magnate who would give…