The Two Ticket Puzzle

A mean-spirited  businessman  Oswald Preston is discovered shot dead under the seat in an empty compartment of a  railway carriage. While there seems to be several people who potentially bore him a grudge including his wife, his  ward, a former employee and his wife's alleged lover a prominent local  doctor. Yet it is almost impossible to see how anyone could have committed the murder while the train was in motion between two  stations.
Ross and his men set about a lengthy, painstaking task of checking all the  train tickets  of those passengers travelling on the train. Eventually, the work pays off and they are able to unmask the murderer and give chase as he attempts to escape.