Trailer Bikes from Adams Trail-a-Bike and Trek. Trailer bike (also known as a trailer cycle or a third wheel, and trademarked names Trailerbike, Trail-a-bike, Half wheeler, Tagalong...) is a one-wheeled (or more rarely two-wheeled) bicycle trailer designed to carry one or more small riders in positions that closely resemble that of a bicycle rider. It can be described as the back half of a bicycle. The rider of a trailer bike usually has a saddle, handlebars, and pedals. Some fold for more compact storage. Our online bike store stocks many new brand name trailer bikes.