あの！すみません、お忘れ物ですよ！ — Oh! Excuse me, you've forgotten something! (Public · Transport, Casual to Stranger, JLPT N4)

Gesture & etiquette

Pointing at the forgotten item. Making eye contact.

Do not touch the item without permission unless absolutely necessary to prevent it from being lost (e.g., if the train doors are closing). Best to alert the owner first.