We don't have ours up under the tree since we use either a G scale christmas train or the O scale Polar Express; however, the diameter of the monorail if you removed the straight tracks would be 49", which is about the size of a G scale train set or about 9" wider than an O-scale. That is plenty of room. In fact, for us, it is a little too large, as it exceeds the circumference of the tree skirt by a few inches. In any event, you do not need more track unless you live at Rockefeller Center or are making an elaborate set up.