car number; car No.
Suffix attached to a number to indicate a specific train car, e.g., 1号車 (car 1). Used in station announcements and on tickets.
お手持ちの指定席券は3号車です。
Your reserved seat ticket is for car 3.
My seat is in car No. 3.
1号車は一番前の車両です。
Car 1 is the frontmost car.
新幹線の5号車に乗ってください。
Please board car No. 5 of the Shinkansen.
Excuse me, which car number is non-reserved seating?
号 is a broad suffix for numbering things like magazine issues or room numbers, whereas 号車 is used only for numbering train cars.
Compound of 号 (number) and 車 (car, vehicle).