A double turnout or tandem is an improvement over a three-throw switch. In a double turnout, turnouts are staggered and take off from the main line at two different places. This eliminates the defects of a three-throw switch, as the heels of the two switches are kept at a certain distance from each other. The distance between the two sets of switches should be adequate to allow room for the usual throw of the point.
Double turnouts can be of similar flexure, when the two turnouts take off on the same side of track (Fig. 15.6) or of contrary flexure, when the two turnouts take off in two different directions (Fig. 15.7).
Double turnouts are mostly used in congested areas, particularly where traffic is heavy, so as to economize on space.