Most 2-rail track systems deliver power to the model trains with a positive current (+) on one rail and a negative (-) on the other. If these currents come in contact with each other directly, a short circuit will result. This poses a problem on track sections where the two rails must cross, including switches (turnouts) and crossings.
In a switch, the point where the left and right rails cross is called the frog, so named for its resemblance to a pair of frog's legs. On both prototype and model track, these frogs can take many forms. On two-rail model tracks, the rails must be insulated from each other at the frog in some manner. Often, the frog is a separate casting and is electrically insulated from both rails by plastic spacers.
Since the frog can't be the right polarity only 50% of the time, it is usually completely insulated. This means there is no power in the frog, creating a dead spot on the railroad.
Locomotives with limited electrical pickups may stall when the wheels cross the frog. Crossovers and yards where switches are lined in rows present an even greater opportunity for loss of current as multiple wheels cross frogs at once.
The Solution:
Power can be added to most switch frogs. The current to the frog must be reversible however to match the polarity (+ or -) of the rail for which the switch is lined. This can be done with a manual switch or an electric relay. Many electric and some manual switch motors and machines have relays built in for this purpose. A third option is to use extra contacts included in many stationary decoders designed for controlling switches through DCC. Controlling the frog polarity simultaneously with the routing of the turnout eliminates the problem of operator error.
