Rail Track Change Specs: Rails, Clips & Turnout Safety

The complex mechanism of a rail track change defines the efficiency and safety of modern railway infrastructure. Also known as a turnout or switch, this system allows rolling stock to move from one track to another, relying on precise engineering tolerances and high-grade materials to withstand immense dynamic loads. For engineers, maintenance teams, and infrastructure planners, understanding the specifications of rail profiles, fastening clips, and turnout geometry is critical for operational success.

Rail Track Change Mechanics and Turnout Geometry

A rail track change is not a single component but a sophisticated assembly of steel parts designed to divert trains. The geometry of this system is dictated by the “frog number,” which determines the angle of divergence.

Frog Numbers and Divergence Angles

The frog is the crossing point where two rails intersect. The angle at which they cross dictates the speed at which a train can safely navigate the turnout.

No. 6 to No. 9 Turnouts: Typically used in freight yards and industrial sidings where speeds are low (5-15 mph). These have sharper angles, creating a more abrupt change in direction.
No. 10 to No. 15 Turnouts: Common on mainlines for crossovers and sidings, allowing for moderate speeds.
No. 20 Turnouts and Higher: Engineered for high-speed lines. A No. 20 turnout, for instance, has a very shallow angle, allowing trains to diverge at speeds often exceeding 45-60 mph without risking derailment or excessive lateral force.

Switch Point Geometry

The switch points (or point blades) are moveable rails that guide the wheel flanges. These are tapered to a fine edge to fit snugly against the stock rail.

Straight vs. Curved Points: Modern high-speed turnouts utilize curved switch points. Unlike straight points, which create a sudden angular change, curved points provide a tangent entry, reducing the impact shock on the wheels and the track components.
Samson Undercut Points: To protect the fragile tip of the switch point, the stock rail is often undercut (machined out) so the switch point can tuck underneath the head of the stock rail. This design, often used in heavy-haul freight (like BNSF or CN networks), prevents wheel flanges from striking the tip directly, significantly extending component life.

The Role of the Stock Rail

The stock rail is the fixed rail against which the switch point rests. It bears the weight of the train and resists the lateral thrust of the wheel flanges. In a rail track change assembly, the stock rail must be machined or bent specifically to accommodate the switch point. It is rigidly braced to prevent it from rolling over under load.

Rail Material Specifications and Standards

The steel used in railway tracks must balance hardness (to resist wear) with ductility (to resist fracture). Specifications typically follow AREMA (American Railway Engineering and Maintenance-of-Way Association) or UIC (International Union of Railways) standards.

High-Strength Rail Grades

Standard carbon rail is suitable for tangent (straight) tracks, but turnouts require premium steel. Heat-treated rail is standard for switch points and stock rails to resist the high impact and friction forces.

Head-Hardened Rail: Through thermal treatment, the head of the rail is hardened to 340-390 Brinell (HB), while the web and base remain more ductile. This is essential for the switch point, which endures sliding friction.
Manganese Steel: Frogs are often cast from Hadfield manganese steel. This material is unique because it work-hardens; the more it is pounded by wheels, the harder the surface becomes, preventing rapid wear at the crossing point.

Rail Section Profiles

Rail is categorized by weight per yard (e.g., 115 lb/yd). Heavier rail offers greater stability and load capacity.

Rail Specification Table

Rail Section	Nominal Weight (lb/yd)	Height (inches)	Base Width (inches)	Head Width (inches)	Moment of Inertia ($in^4$)	Application
115 RE	114.7	6.625	5.50	2.72	65.6	General freight, industrial sidings, yards.
132 RE	132.1	7.125	6.00	3.00	88.2	Heavy-haul mainlines, high-tonnage routes.
136 RE	136.2	7.313	6.00	2.94	94.9	Heavy axle load freight, high-speed corridors.
141 RE	141.0	7.438	6.00	3.12	100.5	Extreme tonnage, premium mainline track.
UIC 60	121.6	6.772	5.90	2.83	73.0	International standard for high-speed rail.

Rail Clips in Track Stability

While the steel rail carries the load, the rail clips (fasteners) ensure the rail remains fixed to the tie (sleeper). In a turnout, clips play a vital role in maintaining the precise gauge required for a safe rail track change.

Elastic Fastening Systems

Modern track infrastructure has largely moved away from rigid spikes to elastic fasteners. These clips act as high-tension springs, holding the rail down with constant force while absorbing vibration.

E-Clips: The most recognizable elastic clip, shaped like a lowercase ‘e’. They are driven into a shoulder cast into concrete ties or welded onto steel plates.

Material: Spring steel bar, typically 20mm diameter.
Toe Load: Generates approximately 2,000 to 2,500 lbs of clamping force per clip.
Application: Universal use in freight and transit.

SKL (Tension) Clamps: Common in European and modern high-speed designs. These use a screw-spike to compress a spring clip.

Advantage: Provides high fatigue limit and allows for fine adjustment of toe load.
Resilience: Excellent resistance to rollover forces in tight curves.

Fastclip Systems: A captive system where the clip is pre-assembled on the tie at the factory.

Efficiency: Allows for automated installation and removal.
Durability: Highly resistant to rail creep (longitudinal movement).

Fastening in Turnouts

Fastening a turnout is more complex than tangent track.

Slide Chairs: Under the switch points, standard tie plates are replaced by slide chairs. These usually do not use standard clips on the gauge side (to allow the switch rail to move). Instead, the stock rail is braced on the field side using adjustable rail braces.
Rail Braces: These are heavy-duty supports bolted or welded to the switch plates. They push against the web and underside of the head of the stock rail to prevent it from tilting outward when the wheel flange strikes the switch point.
Gauge Plates: At the very tip of the switch, insulated gauge plates connect the two stock rails. These ensure the distance between the rails (gauge) remains exactly 56.5 inches (standard gauge), preventing the rails from spreading and causing a derailment.

Turnout Components and Design Requirements

A turnout consists of four main functional areas: the switch, the closure, the frog, and the guard rails.

Switch Stand and Throw Mechanisms

The force to move the switch points comes from the switch stand or a powered switch machine.

Throw Distance: The gap between the open switch point and the stock rail must be sufficient to ensure wheel flanges do not clip the back of the open point. A standard throw is typically 4.75 inches at the No. 1 rod.
Switch Rods: These connect the two switch points so they move in unison. They are insulated to prevent shorting out signaling circuits (track circuits). The No. 1 rod connects directly to the throw mechanism.

Heel Blocks and Closure Rails

The switch point pivots at the “heel.”

Heel Blocks: These are cast steel spacer blocks placed between the switch rail and the closure rail. They act as a hinge, maintaining the correct spread while allowing the flexibility needed for the switch to throw.
Closure Rails: These connect the heel of the switch to the toe of the frog. In high-speed turnouts, these must be carefully curved to match the turnout geometry to ensure a smooth ride.

Guard Rails (Check Rails)

Located opposite the frog, guard rails are a critical safety feature.

Function: They engage the back of the wheel flange to pull the axle assembly away from the frog point. This ensures the wheel on the opposite side passes safely through the flangeway of the frog without striking the point.
Check Gauge: The distance from the face of the guard rail to the face of the frog is a critical safety dimension. If this distance is incorrect, wheels can “pick” the frog point, leading to derailment.

Safety Standards for Rail Track Change Operations

Because turnouts represent a discontinuity in the rail, they are the most maintenance-intensive parts of the track structure. Regulatory bodies like the FRA (Federal Railroad Administration) and Transport Canada set strict limits on wear and geometry.

Inspection Tolerances

Point Gap: When the switch is closed, there must be zero gap between the switch point and stock rail. A gap of just 1/16th to 1/4th of an inch is enough for a worn wheel flange to climb the rail or split the switch.
Wear Limits:

Switch Points: Excessive side wear or chipping on the point is dangerous. If the top surface of the point is worn lower than the stock rail, it cannot lift the wheel wheel tread, creating a derailment hazard.
Frogs: Manganese frogs are prone to cracking and crushing. Automated inspection and regular welding repairs are required to maintain the surface profile.

Lubrication and Winterization

Slide Plates: The slide chairs under the switch points must be lubricated (graphite or synthetic grease) to ensure the switch throws smoothly with minimal force.
Heaters: In cold climates, snow and ice can pack between the switch point and stock rail, preventing the switch from locking. Gas or electric switch heaters (blowers or radiant rails) are mandatory specifications for mainline turnouts in northern regions.

Derails and Protection

In industrial sidings, rail track change mechanisms are often paired with derails. A derail is a safety device designed to intentionally derail rolling stock to prevent it from rolling uncontrolled onto a mainline.

Split Point Derails: These function exactly like a single switch point, guiding a runaway car off the tracks.
Crowders: Often used with sliding derails, these push the wheel flange into the derail block to ensure positive engagement.

Frequently Asked Questions

How do trains change tracks without steering?
Trains rely on wheel flanges and the turnout mechanism. The switch points guide the flanges; if the switch is set to diverge, the curved point pushes the flange, steering the entire axle onto the new track. The wheels passively follow the rails.
What prevents a switch from moving under a train?
Switches utilize a locking mechanism (facing point lock) or hydraulic clamp. Once the points are set, a locking bar engages to mechanically hold the rails against the stock rail. Signal circuits also prevent the switch motor from operating if a train is detected over the switch.
What is the difference between a frog and a switch?
The switch is the movable part at the entry of the turnout that initiates the turn. The frog is the rigid crossing point further down where the rails intersect, allowing wheels to cross the opposing rail to reach the diverging track.
Why are some rail track changes rougher than others?
This depends on the “frog number.” A low number (e.g., No. 8) has a sharp divergence angle, causing a jerky lateral motion. High numbers (e.g., No. 20) have long, gradual tapers, allowing smooth transitions at higher speeds.
Can a train run through a closed switch from the wrong direction?
Generally, no. This is called “running through” a switch and will damage the locking mechanism and bend switch rods. However, “trailable” switches exist in some transit yards that use springs to allow wheels to push the points over without damage.