The golden train ticket office is outside the train station building. Here's a picture of it to save you from running around the big building with Ga Saigon written on it like I did.
Pick up your tickets 1 hour prior to departure. Board the train 30 minutes prior to departure.
I really liked using the overnight trains in Vietnam. Leaves the boring travel to your sleep time, saves you a hotel and wake up at your next destination. The beds are simple but comfortable enough.
