A supplemental ticket required in addition to the basic fare to ride special express trains (e.g., limited-express). Often called a 'tokkyuken' (express ticket).
窓口で乗車券と特別急行券を一緒に買った。
I bought the basic fare ticket and the special-express ticket together at the counter.
この列車に乗るには、別途特別急行券が必要です。
A separate special-express ticket is required to ride this train.
指定席を取るなら、特別急行券と指定席券の両方を確認してください。
If you're reserving a seat, please check both the special-express ticket and the reserved seat ticket.