In the novel The Mystery of the Blue Train, Katherine Grey is an ex-companion to Mrs Harfield, to whom the other’s wealth is left in her will. She travelled on Le Train Bleu to the French Riviera. Katherine is a cousin of Rosalie Tamplin, which lives in Villa Marguerite. She is one of the Worcestershire Greys.
She lives in St. Mary Mead, Kent.
Agatha Christie's Poirot
In the "The Mystery of The Blue Train" episode of Agatha Christie's Poirot, Katherine Grey was portrayed by Georgina Rylance. Here a subplot is added in which Rufus Van Aldin had led Katherine's father to suicide. This was probably made up so that she had a valid motive for killing his daughter.
At the end of the episode, Katherine says she discovered to be very fond of traveling, and said that she would have boarded the Orient Express at Vienna. This could be an allusion to the events of Murder on the Orient Express (note that, in Murder on the Orient Express, Katherine Grey does not appear, nor she does in any adaptation of the novel). In the original The Mystery of the Blue Train, there is a different connection between the two novels. The attendant of Le Train Bleu is named Pierre Michel, just like the one on Murder on the Orient Express. For more information about the two characters, visit this page.