common crossing

A common crossing is the component in a railway turnout where two rails physically cross each other at an angle. It is also called a frog, a term that persists in track work despite sounding archaic. The crossing is the point where a train moving on the diverging route passes through the main line, or vice versa, and both sets of rails must occupy the same space.

The crossing is necessarily a complex casting or forging because it must support wheel loads while accommodating the angular intersection. The two rails approach each other, meet at a point (the apex or nose of the frog), and diverge again. At this crossing point, wheels roll directly over the intersection without benefit of continuous rail support, which is why the component must be exceptionally rigid and precisely aligned. The angle of the crossing determines the gradualness of the track divergence: a No. 10 crossing has an angle of roughly 5.7 degrees, while a No. 20 crossing narrows to about 2.9 degrees.

Crossings are manufactured from steel castings or ductile iron, with the wearing surfaces hardened to resist the repeated impact of wheels. The throat of the crossing (the space between the two rails) is kept as narrow as practically possible to minimize wheel drop-through and lateral movement. Over time, the nose of the crossing wears and can become chipped or cracked under heavy tonnage, particularly at locations with high-speed or heavy-load traffic. Once damaged, the entire crossing assembly typically must be replaced rather than repaired.

The common crossing sits at the center of any turnout, flanked by switch points at one end and guard rails on either side. Unlike movable switch points that direct a train to one route or another, the crossing itself is stationary: all trains pass through it regardless of which way the points are set. This is why crossing maintenance directly affects the safety and smoothness of track geometry across both the main and diverging lines. Railroads classify crossings by their number designation, with higher numbers indicating gentler angles and longer crossings suitable for higher speed traffic.