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first departure (of the day); first train; first bus
departing from the starting station (of a bus, train, etc.)
出張のため明日は始発電車で空港に向かう。
For my business trip, I'm heading to the airport on the first train tomorrow.
始発駅から乗れば必ず座席を確保できる。
If you board from the first station, you can definitely get a seat.
始発に乗り遅れないよう目覚まし時計をセットした。
I set the alarm so I would not miss the first train.