frog

A frog is a mechanical component mounted on the overhead catenary wire of a streetcar or trolley system. Its purpose is to guide the trolley pole smoothly from one wire to another when the vehicle approaches a track switch. The frog sits at the junction point where two wires meet or diverge, and it uses a sliding or pivoting action to direct the pole down the correct wire path without disconnecting electrical contact.

The device gets its name from the frog rail component used on track itself, a similarly shaped casting that allows flanged wheels to cross from one rail to another. Just as a rail frog routes the rolling stock, the overhead frog routes the current collector. Early frogs were simple V-shaped metal guides; modern versions use more sophisticated geometries to handle higher speeds and heavier poles without arcing or damage.

Frogs are typically made of bronze or forged steel and are bolted or clamped directly to the wire or to a support bracket. The trolley pole's wheel or grooved contact roller rides along the frog's surface as the car moves through the switch zone. Proper frog design prevents the pole from derailing sideways or losing electrical continuity, which would cut power and bring the vehicle to a halt.

Common failure modes and maintenance

Frogs wear under constant mechanical friction and electrical arcing, especially at high-traffic intersections or on systems with frequent switching. Wear creates gaps and flat spots that cause rough transitions, increased sparking, and eventual pole derailment. Regular inspection for corrosion, cracks, and surface deformation is essential. Some systems require replacement of frogs every few years depending on traffic volume and maintenance discipline.

Frog design varies by wire configuration: single-wire systems use simpler designs, while multi-wire crossover points demand more complex geometries. Many heritage and light-rail systems still rely on original frogs from the early-to-mid twentieth century, though some have been retrofitted with stainless steel or hardened bronze surfaces to extend service life. Proper lubrication of the contact surface reduces wear, but excessive lubrication can increase slipping and reduce traction.