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.
Your reserved seat ticket is for car 3.
Car 1 is the frontmost car.
My seat is in car No. 3.
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).