High Quality Content by WIKIPEDIA articles! A flange bearing frog, often abbreviated FBF, is a type of frog in which the flange of the wheel on a railway vehicle supports the weight of the vehicle. In conventional practice, the tread of the wheel rests on the head of the rail and bears the weight of the vehicle, while the flange is used to keep the vehicle in the gauge of the track. Flange bearing frogs are a relatively recent development, as a means to reduce maintenance costs associated with turnouts and diamonds, where rails must cross one another.