Famous Five Colour Short Stories: Five and a Half-Term Adventure

Julian, Dick, Anne, George and Timmy the dog find excitement and adventure wherever they go in Enid Blyton's most popular series. A short story in colour for younger readers.
George's dog Timmy sniffs out an adventure when he spots some suspicious-looking passengers on a train. He is very interested in one of them, but what has he spotted?
Can the Famous Five solve this mystery?