Frog

A frog is the V-shaped casting at the heart of every turnout where two rails cross at an angle, allowing the wheels of a train to traverse from one route to the other. The name almost certainly comes from the original British shape of the casting — a forked piece of iron that, viewed from above, resembled a sprawling frog — and has stuck across the English-speaking railway world long after the original casting style was replaced by modern manganese-steel monoblocs.

The frog has two critical surfaces: the point (where the wing rails converge to a sharp V) and the throat (the gap between the converging rails immediately ahead of the point). Because the gap interrupts the continuous running rail, a wheel briefly loses support over the frog and is guided across the gap by a check rail running opposite the frog on the inside of the wheelset. The geometry has to be exact: too narrow a gap and the wheel flange catches the point, derailing; too wide and the wheel drops into the throat. Frog angles are described as 1-in-N numbers (1-in-8, 1-in-12, 1-in-20), with higher numbers indicating a shallower angle and a faster permissible speed through the diverging route.

Two engineering refinements address the frog's wear problem. Spring frogs use a movable wing rail that's normally pressed against the main route, eliminating the gap for trains on the through line. Swing-nose frogs — standard on high-speed lines worldwide — go further by physically moving the entire point assembly so that a true continuous rail exists for the route being signalled, at the cost of a more complex actuator and a tighter maintenance regime.

For railfans, a frog is the signature audible feature of a freight train traversing a yard throat: the rhythmic clack-clack as each wheelset crosses the gap is what makes a switching move sound different from a clean mainline run.