A "train" is the veil that goes down a bride's back and not in front of her face. I'm guessing it's called a train because it "trails" after the bride when she walks down the aisle. Pagegirls are little girls designated to holding the bride's train on the day of her wedding.

She's talking about a wedding dress, clearly. The long piece of cloth that trails behind her as she walks is called the "train" (of the dress). This is usually carried by one pagegirl (a very young girl - definitely less than 10 years old, usually much younger), so the train does not touch the ground. This particular silk train is very long, so the bride needs twelve pagegirls (six each side) to help keep the cloth off the ground.