The point at which the left and right rails cross in a switch or turnout is called the frog. The term comes from the similarity in appearance to a pair of frog's legs. On both models and the prototype, frogs can be a single solid casting or formed from bends in the intersecting rails.
Frogs are also found in diamonds or crossings where trains cannot change paths. Typically these frogs are noted by the degree of the angle of the crossing. Each crossing will include four frogs.
There are many different styles and sizes of frogs on both the prototype and model railroads. The radius or degree of the curve will determine the size of the frog. Frogs are numbered corresponding to the size of the turnout, with higher numbers denoting larger frogs and a more gentle curve.
The number of the frog corresponds to the length of run needed for the rails to diverge a distance of one unit. So a Number 5 frog would result from a switch where the rails were 1 inch apart after a run of 5 inches. The unit of measure is not important as long as the measurements are consistent.
Wye turnouts have a number that is half that of a normal left or right turn out because both tracks are diverging concurrently. So a "No. 3 Wye" is the equivalent of a No. 6 standard turnout.
How Model Train Layouts Use Switch Frogs
Typical model train layouts use switches with frogs between a No. 4 and No. 10. Frogs of this size would be reserved for yards and tight industrial track applications on the prototype.
Most model railroad curved turnouts are not numbered either. These are typically listed by their two radii. Some manufacturers simply use generic terms like "large radius" to denote their different sizes.
On really large prototype turnouts, some frogs are so long that they have a set of points of their own. These "movable point frogs" require a second switch motor or mechanical linkage from the main set of point rails which change the train's route. Some model train switches use the same technology, particularly in S Gauge where the need for large flange ways makes a movable point frog necessary.
On most two-rail model tracks, the frog must be insulated to prevent an electrical short between the positive and negative rails. It creates a dead-spot on most commercial turnouts. Frogs can be powered with the use of an electrical relay or switch. These relays are built into many of the switch machines available. It makes powering the frog as simple as connecting three wires.
For model railroads equipped with DCC, an insulated frog is also important to prevent even minor short circuits which could harm the command system. Typically this affects switches which don't use a separate frog casting. Metal wheels with a broad face could make contact with both rails simultaneously, creating a short. Older turnouts so affected are usually easy to correct by cutting insulating gaps around the frog and wiring it as described above.
Dual gauge turnouts and 3-rail track systems require additional frogs in each switch to accommodate the extra rails. Other oddities include slip and double-slip switches which are a combination of a turnout and crossing, and gauntlet tracks. Gauntlets use a frog at each end to allow two tracks to overlap as they pass through a narrow area like a bridge or tunnel. Although two trains still can't pass at the same time, the extra costs of a full pair of switches into a single track is eliminated.