Except for right-angled crossings, each one is designed for a particular location.
A frog is needed at each intersection to enable the wheel flanges to pass along both tracks. For flat-
angled crossings, conventional frogs are used at the ends but specially designed ones in the center. In figure 2.21,
the end frogs are lettered (A) and the center ones (B). The end frogs in the crossing shown in figure 2.21 are the
rigid type. But when they are used in such a wide-angled crossing, the flangeway openings are also necessarily
wide. These are dangerous and cause a rough ride and increased point damage. To avoid these bad features,
crossings are usually built with movable-point frogs. A track crossing with such frogs is illustrated in figure 2.22.
The movable points consist of two knuckle--angled--rails against which rails mitered to the crossing angle and its
supplement are moved to form a solid wheel support or opened to provide the flangeway needed for moving over
opposing routes. In figure 2.22, the knuckle rails are labeled and the crossing angles are marked (A). Movable-
point frogs act somewhat like switches in that they must be set for one track or the other.
Figure 2.21. Track Crossing With Rigid Frogs.
Figure 2.22. Track Crossing With Movable-Point Frogs.