Formal — to staff
Shinkansen · Seat · Issue
You
あの、恐れ入りますが、ここ、何号車の何番のお席ですか？
あの、おそれいりますが、ここ、なんごうしゃのなんばんのおせきですか？
Excuse me, but may I ask which car and seat number this is?
Romaji: Ano, osoreirimasu ga, koko, nangousha no nanban no oseki desu ka? / Reply Romaji: E, a, sumimasen! Moshikashite watashi, gousha wo machigaetemasu ka? Kakunin shimasu ne.
Reply
えっ、あ、すみません！もしかして私、号車を間違えてますか？確認しますね。
えっ、あ、すみません！もしかしてわたし、ごうしゃをまちがえてますか？かくにんしますね。
Oh! I'm so sorry! Did I get the car number wrong? Let me double-check.
Gesture & etiquette
Hold your own ticket with both hands and bow your head slightly while asking in a soft voice.
Do not directly accuse the person by saying 'You are in my seat.' Asking for their seat and car number politely prevents unnecessary tension and avoids embarrassment if you are actually the one in the wrong car.