What is a Railway Switch (Turnout)? How Trains Change Tracks

On 3 June 1998, the Eschede ICE disaster killed 101 people — the deadliest high-speed rail accident in history. The immediate cause was a fractured wheel tyre. But the mechanism of catastrophe was a railway switch: the derailed wheel struck the frog of a switch at 200 km/h, causing the entire switch assembly to be lifted and flipped onto a road bridge above the track. The bridge collapsed onto the passing train.

Switches are among the most mechanically complex and safety-critical components in railway infrastructure. Every train movement through a junction, every entry into a station, every crossing of a parallel track depends on a switch working correctly. Understanding how switches work — and why high-speed switches are so different from conventional ones — is fundamental to understanding railway operations and the engineering challenges of high-speed rail.

What Is a Railway Switch?

A railway switch is a mechanical installation that enables a train to be guided from one track to another. Unlike road vehicles, trains cannot steer — their flanged wheels follow the rails. The only way to change a train’s direction is to change the track itself.

A switch achieves this by incorporating a pair of movable switch rails (also called point rails or blades) that can be positioned against either of two fixed stock rails. In one position, the switch rails guide the wheel flange straight ahead onto the main line. In the other, they divert the train onto a diverging track. The switch is operated remotely by a point motor (electric or hydraulic actuator) connected to the interlocking system.

Key Components of a Railway Switch

The Frog: The Most Critical Component

The frog — named for its resemblance to a frog’s foot — is the crossing point where two rails intersect. It is the most mechanically challenging part of any switch, and the primary source of noise, vibration, and maintenance cost.

The problem is geometric: at the crossing point, one rail must cross the other. This creates a gap in each rail where the crossing rail passes through. Every wheel travelling over the frog must cross this gap — a moment when the wheel tread loses rail support and drops slightly before the wheel flange or the opposite rail picks it up again. This gap creates impact, vibration, and wear, and is the reason switches have speed restrictions on the diverging route.

The frog angle — expressed as a ratio (1:9, 1:15, 1:65) — describes the geometry of the divergence. A 1:9 frog has a relatively sharp divergence angle (suitable for low-speed yard work); a 1:65 frog has a very shallow angle (used on high-speed lines for gentle divergence at high speed). The larger the second number, the longer the switch and the higher the permitted speed.

Fixed Frog vs Movable Point Frog: The High-Speed Solution

Switch Types: A Complete Guide

The Frog Number: Understanding Switch Geometry

The frog number (also called the crossing number or turnout ratio) is the key parameter describing switch geometry. It is expressed as 1:N, where N is the cotangent of the crossing angle — the number of units of straight travel for every unit of lateral divergence.

How Points Failures Cause Delays

Switch failures are one of the most common causes of railway delays. A single failed switch at a busy junction can block multiple routes simultaneously, since many trains share the same switch for different movements. The failure modes include:

• Detection failure: The point motor moves the switch rails, but the detection circuit does not confirm that the rails have seated correctly against the stock rail. The interlocking prevents any train movement over the switch until the fault is cleared.
• Mechanical jam: Ice, debris, or a foreign object prevents the switch rails from moving to the correct position. In winter, switch heating failures are a major operational risk.
• Broken switch rail: A fatigue fracture or impact damage to a switch rail requires immediate possession and rail replacement — typically a multi-hour process.
• Motor failure: The point motor fails mechanically or electrically, requiring either emergency manual operation or motor replacement.

On high-density networks, a single switch failure at a major junction can affect dozens of trains per hour. Network Rail estimates that points failures account for approximately 14% of all infrastructure-related delay minutes on the UK network — a disproportionate impact for components that represent a small fraction of total track infrastructure.

Smart Switches: Digital Monitoring and Predictive Maintenance

The traditional approach to switch maintenance is time-based: inspect at fixed intervals, replace components on a schedule. Modern railway operators are shifting to condition-based maintenance using embedded sensors that monitor switch health continuously:

• Current signature analysis: The electrical current drawn by the point motor as it operates forms a characteristic signature. Changes in this signature — increased current indicating increased resistance, or abnormal timing — can detect developing mechanical problems before failure.
• Force measurement: Strain gauges on stretcher bars measure the force required to operate the switch, detecting early signs of rail distortion or debris accumulation.
• Temperature monitoring: Sensors detect heating system failures before they cause icing problems.
• Vibration analysis: Accelerometers on switch components detect unusual vibration signatures that may indicate loose fastenings or component wear.

Siemens Mobility’s Simis IS and Alstom’s SmartLock systems, among others, integrate these sensor streams with predictive algorithms to identify switches at elevated failure risk, enabling proactive intervention during planned possessions rather than reactive response to failures during traffic hours.

Editor’s Analysis

Frequently Asked Questions

Q: What is the difference between “points,” “switch,” and “turnout”?

These are regional terms for the same component. “Points” is the standard British English term, used by Network Rail and throughout the UK rail industry. “Switch” is used in both British and American English to describe the movable rails specifically, but is also used informally to refer to the complete assembly. “Turnout” is the standard North American term for the complete assembly including switch rails, closure rails, and frog. In European standards and international rail engineering, “switch” and “turnout” are both used, with “switch” often referring to the blade/rail portion and “turnout” to the complete installation.

Q: Why do trains slow down through switches?

Trains slow down on the diverging route through a switch for two reasons. First, the curved geometry of the diverging path imposes a lateral acceleration on passengers and cargo — faster speeds through the curve create greater discomfort and higher wheel-rail forces. Second, the frog gap creates an impact as each wheel drops briefly into the gap, and at higher speeds this impact is more severe, increasing wear and the risk of derailment. High-speed switches with movable frogs address the second issue by eliminating the gap, but the first issue — the curve geometry — still limits diverging speed regardless of frog type.

Q: What is a derailment switch and why is it used?

A derailment switch (also called a catch switch or trap point) is a deliberately designed safety device that causes a runaway or unauthorised vehicle to derail in a controlled manner before it can enter a main line or collision with another train. They are placed at the end of sidings, on gradients where a vehicle could roll away, and at the entrance to occupied platforms. The derailment switch is normally set to the derailing position and must be positively set to “safe” before any authorised movement. If a vehicle runs away without authorisation, it strikes the derailment switch and is guided off the track at low speed into a sand drag or buffer stop — a controlled derailment that prevents a potentially catastrophic main line collision.

Q: How are switches operated and who controls them?

Most mainline switches are operated remotely by the signaller (train controller) via the interlocking system — a safety system that ensures switches are correctly set and locked before a signal is cleared for a train to proceed. The interlocking prevents conflicting routes from being set simultaneously. Point motors (electric or hydraulic actuators) physically move the switch rails and send electrical confirmation back to the interlocking when the rails are correctly seated. On some secondary lines and depots, switches may still be operated manually by a trackside lever, but this is increasingly rare on mainline infrastructure.

Q: What happens if a switch is set against a moving train?

If a switch is moved while a train is passing over it — an event called “running through” or “splitting the points” — the results depend on speed and the type of switch. At low speeds, the train may continue through without derailing if the switch rails are not moved far enough to deflect the wheels. At higher speeds, the wheel flange may be caught by the moving rail and deflected off the track, causing a derailment. Modern interlocking systems prevent this by locking switches in position during route occupancy and by detecting train presence on the switch before allowing any movement. “Run-through” derailments are rare on signalled mainlines but can occur in depots or on unsignalled sidings.