What Is a Rail Turnout
A rail turnout is the track structure that guides a train from one line to another, from the main track into a siding, or from one track of a station throat into another. It consists of two key functions working together: the switch moves the running edge so the wheel set is directed onto the intended route, and the frog provides a continuous crossing for wheel flanges where the two rails intersect. Turnouts are the most maintenance-intensive part of plain track because every wheel passage through the switch and frog applies concentrated impact and lateral forces. Their design, rail sections and fastenings are governed by dedicated standards rather than by plain track rules: the EN 13232 series for switches and crossings in Europe, the AREMA manual for North America, and the corresponding national rules elsewhere. The number of a turnout, its switch geometry and its frog angle together set the maximum speed at which a train can use the diverging route.
Main Components of a Turnout
A standard single turnout is made of three functional parts. The switch at the beginning consists of two stock rails and two movable switch rails, also called points, driven by a switch machine through rods and stretcher bars. When the machine moves the switch rails, one switch rail closes against its stock rail while the other opens, and the wheel flange is directed either straight ahead or onto the diverging route. The connecting part, or lead, is the curve that links the switch to the crossing, with sleeper spacing gradually changing and additional fastening components such as gauge tie rods holding the two rails to gauge. The crossing at the far end contains the frog, where the two running edges of the rails intersect, flanked by guard rails on the opposite rails, which control the wheel flange so it cannot strike the frog point or take the wrong route. A simple turnout thus guides the wheel through three distinct phases, and each phase has its own wear and geometry requirements.
Turnout Types and Numbering
Turnouts are classified by the number of routes they join. A single turnout divides one track into two; a double turnout gives two diverging routes from one stem; a three-way turnout uses two successive switches in one length and is used where space is short. Scissors crossings and double slips combine several turnouts in one compact layout. The governing parameter of a turnout is its number N, defined as the cotangent of the frog angle, N = cot(alpha), where alpha is the angle between the through route and the diverging route. A smaller frog angle gives a larger number and a shallower crossing, which allows higher speed on the diverging route. Common practice is No. 9 for slow sidings, No. 12 for conventional mainline diverging moves, and No. 18, No. 42 and higher for passenger-dedicated and high-speed lines. Typical permissible diverging speeds are about 30 km/h for No. 9, 45 km/h for No. 12 on conventional lines, around 80 km/h for No. 18 on passenger-dedicated lines and about 160 km/h for No. 42 on high-speed networks; always confirm against the owning authority, because the permissible speed also depends on curvature and switch geometry, not on the frog number alone.
Frog and Guard Rail Design
The frog is the crossing where the flange routes meet, and its design decides turnout life. A fixed frog is a cast or built-up crossing with a fixed point, machined from austenitic manganese steel in heavy-duty designs because the work-hardening surface gives outstanding wear resistance. Fixed frogs are simple and cheap, but the flange gap at the point creates an impact each time a wheel crosses, which limits speed and drives surface batter. A movable-point frog closes the gap: the point moves with the switch so there is no flange gap and wheels cross smoothly, which is why movable-point frogs are standard for high-speed turnouts above about 160 km/h. Guard rails, or check rails, are set opposite the frog to force the wheel flange into the correct flangeway and prevent the wheel from striking the frog point or climbing the crossing. The flangeway width is a design-critical dimension: too wide and the wheel can hit the point; too narrow and the flange binds. Dimensioning follows the national turnout standard, and in Europe the EN 13232 series defines the geometry, tolerances and test requirements.
Turnout Fastenings and Track Support
Turnouts place higher loads on fastenings than plain track because of the lateral forces on the switch rails and the impact at the frog. Clips in the switch area must hold the stock rail while leaving the switch rail free to move, so switch-area fastenings differ from plain track fastenings: switch rail is fastened on the gauge side with slide chairs or baseplates that allow the rail to slide, while the stock rail is positively held. Gauge tie rods connect the two rails across the track to maintain gauge against lateral thrust. Sleeper spacing is reduced through the turnout, often from 600 mm on plain track to 550 to 600 mm or closer through the crossing, to distribute the higher loads. Rail pads under the frog and through the crossing are specified for the higher impact, and elastic fastenings are used throughout except where the slide chair function requires sliding support. Ballast depth and compaction are critical under turnouts, since differential settlement through the crossing is a frequent cause of geometry faults.
Common Problems and Maintenance
Turnout maintenance concentrates on four faults. Switch rail side wear, where the switch rail is worn by flanges on the closed side, reduces the rail section and eventually requires switch rail renewal. Frog batter, the flattening and cracking of the crossing surface from repeated impact, is repaired by grinding on fixed frogs within limits, and by replacement when the running surface falls below the safety limit. Gauge faults from loose gauge tie rods or sleeper settlement change the flangeway and can cause wheels to strike the frog point. Geometry faults from differential settlement produce twist and poor riding. The inspection routine therefore checks switch rail wear, frog surface condition, flangeway and guard rail gauge, fastening torque and sleeper condition at a frequency several times that of plain track. When a turnout is specified, buyers should provide the frog number, the rail section, the switch geometry and the permissible speed, because the frog angle and switch type drive the cost and the maintenance profile of the whole unit.
Frequently Asked Questions
Q1: What is the difference between a switch and a turnout?
A switch is the movable part of a turnout, the stock rails, switch rails and switch machine that guide the wheel onto the route. The turnout is the complete structure including the connecting lead and the frog. In everyday speech the words are often used interchangeably.
Q2: What does the number of a turnout mean?
The number N equals the cotangent of the frog angle. A No. 12 turnout has a frog angle whose cotangent is 12, about 4.8 degrees. Larger numbers mean shallower crossings and higher permissible diverging speed.
Q3: Why do high-speed turnouts need movable-point frogs?
At high speed the flange gap of a fixed frog creates an impact that damages wheels and rail and limits speed. A movable-point frog closes the gap, so wheels cross smoothly, which is why it is standard on lines with diverging speeds above about 160 km/h.
Q4: What are guard rails for in a turnout?
Guard rails are fixed opposite the frog to control the wheel flange in the flangeway, preventing the flange from striking the frog point or climbing onto the wrong route. They are essential safety components of every crossing.
Q5: Why do turnout rails wear faster than plain track?
Because every wheel passage through the switch and frog applies lateral forces and impacts that plain track does not see, and because the flangeway and switch openings concentrate contact stress on small areas of the running surface.
Q6: Which standards govern turnout design?
In Europe the EN 13232 series covers switches and crossings; in North America the AREMA manual does. National rules add the frog numbering and permissible speeds. Buyers should state the frog number, rail section and speed requirement when requesting a turnout quotation.