Formal — to staff
Public · Transport
すみません！今出発した電車（バス）にパスポートケースを忘れてしまいました！〇両目（〇番席）の窓側の席です！ — Excuse me! I left my passport case on the train (bus) that just departed! It was on the window seat of car number [X] (seat number [X])! (Public · Transport, Formal — to staff, JLPT N4)
You
すみません！今出発した電車（バス）にパスポートケースを忘れてしまいました！〇両目（〇番席）の窓側の席です！
すみません！いましゅっぱつしたでんしゃ（バス）にパスポートケースをわすれてしまいました！〇りょうめ（〇ばんせき）のまどがわのせきです！
Excuse me! I left my passport case on the train (bus) that just departed! It was on the window seat of car number [X] (seat number [X])!
Romaji: Sumimasen! Ima shuppatsu shita densha (basu) ni pasupōto kēsu o wasurete shimaimashita! Maru-ryōme (Maru-banseki) no madogawa no seki desu! / Reply Romaji: Dochira no rosen, nanji nanpun no bin desu ka? Sugu ni ekiin (untenshi) ni renraku shimasu.
Reply
どちらの路線、何時何分の便ですか？すぐに駅員（運転士）に連絡します。
どちらのろせん、なんじなんぷんのびんですか？すぐにえきいん（うんてんし）にれんらくします。
Which line and what time was the service? I will contact the station staff (driver) immediately.
Gesture & etiquette
Urgent hand motion, point in the direction the train/bus went.
Do not try to run after the departing vehicle or block its path. Provide as much detail as possible (time, line, seat).