n. when you have braces on the top and bottom row of your teeth, giving the wearer the look of having mini traintracks running through their mouth.
the gaps between your teeth are the bars and your braces are the tracks.
rebecca: guess who's getting traintracks?
rebecca: simon! he's also going to the same orthodontist as me!
courtney: doesn't mei go to that one too?
rebecca: oh, whoops...