電車の中にリュックを忘れてしまったのですが... — I forgot my backpack on the train... (Lost · and · Found · Transport, Formal — to staff, JLPT N4)

Gesture & etiquette

Holding hands to head as if stressed, pointing in the direction a train might have gone.

Provide as much detail as possible: line name, direction, specific car number if known, time of travel, and detailed description of the item. Be polite and patient.