A train bitch is a person who follows a celebrity around arranging her train for photos. This is less so to prevent a person from tripping than to make sure it looks just right for the cameras. As her train bitch, Beyoncé employed a dude wearing sequin harem pants (not Justin Bieber, no) who I was told was her stylist. He had to flatten out her train because it was cut at the end in some kind of elaborate Baroque shape, and this had to be evident in all photos. (God forbid, no unflattering photos.) Rooney Mara also had a train bitch, but hers wasn't wearing sequined harem pants and was therefore less memorable.
(I feel compelled to note Heidi Klum had a purse bitch, but she was wearing pastel blue peplums and her purse was a feathery pouf of a thing, so none of it was punk at all.)