Follow Me Young Man(1958)

Original title: Suivez-moi jeune homme

Françoise, a charming young saleswoman for a leading jeweller, is being courted in vain by a billionaire shipowner. He attributes her failure to the existence of a rival, and decides to have her followed by a private detective. Françoise, commissioned by her boss to transport a valuable necklace to Cannes, is accompanied by a bodyguard charged with her safety. Françoise, the bodyguard and the detective meet on the train and a series of misunderstandings ensue.