CBTC Secondary Systems

Communications Based Train Control (CBTC) uses telecommunications to continuously determine the position of a train, with greater accuracy compared to conventional signaling systems. CBTC equipped trains communicate directly with trackside equipment, improving traffic flow by allowing shorter operating distances between trains. 

When operating as designed, a CBTC system helps transits keep their lines running smoothly and efficiently. However, there are certain situations in which proper CBTC system function may be interrupted, requiring a backup system. A backup may also be required per CBTC design to handle non-equipped cars that enter an equipped area.

General benefits of a secondary train detection system

Automatic protection for trains not equipped with CBTC equipment
A CBTC secondary train detection system will detect trains and other vehicles to determine if a track section is occupied or clear, including those not equipped with CBTC equipment. Examples include maintenance vehicles or non-CBTC equipped trains that use the same tracks. The CBTC secondary system can then allow for a safe separation distance between vehicles.

Protection for CBTC equipped trains when communication is disrupted
There are several scenarios that can cause safety concerns or system delays with a CBTC system. If a system is partially down for any reason, such as an equipment failure on even a single train, the system will go into fail-safe mode. Train movement is eventually resumed under manual operating procedures. By using a CBTC secondary system, automatic train tracking and movement will continue, albeit with longer headways, in a vital, safe manner.

Protecting entry and exit points of CBTC & non-CBTC territory
Trains frequently move between CBTC protected and non-CBTC areas. An example is when trains move across the boundary from an unmonitored rail yard or maintenance facility to a CBTC protected area. A CBTC secondary system will monitor the entry and exit of vehicles at the boundaries of CBTC territory.

Challenges for conventional technology as secondary systems

Installation costs and time (greenfield projects)


Installing conventional signaling technology as a CBTC secondary system is expensive and time consuming. Drilling of the rail and installation of insulated joints, bonds, and wiring is required, as well as extensive and frequent maintenance over the lifetime of the equipment.

Frauscher Benefits

Quick, low cost installation without modification of track layout 

Frauscher axle counting systems can easily integrate with the operator’s existing signaling technology, without modifications to track or layout. Installation is quick, requiring just five minutes per wheel sensor. The sensor is simply clamped to the rail using the Frauscher rail claw, with no drilling required. This is particularly beneficial for brownfield projects, as track access time to complete the work would be limited.

Upgrading aging components and ongoing maintenance costs (brownfield projects)


An operator will often decide to invest in a CBTC system when the existing system is nearing the end of its useful life, or when an increase in capacity is needed. Overlaying or replacing old track circuits with new ones, or re-using the existing signaling layout, creates a major challenge and expense. The existing signaling layout might not fit the needs of the CBTC system for several reasons, such as limited block lengths or cross overs that have dead areas.

If the operator decides to reuse the existing train detection solution, complicated cut over procedures must be implemented when testing or in revenue service operation. This creates additional risk by touching or interfering with the vital, existing system each time testing is conducted. Additionally, new track circuits can be nearly impossible to overlay with the existing technology.

Installation and maintenance work required with conventional systems must be completed during off times, which is more expensive. Significant costs will be incurred over a new track circuit system’s lifetime to maintain bonds, joints, and wiring, which must be inspected regularly. This requires more worker time on track and a potentially negative impact on safety. 

Frauscher Benefits

Quick, low cost installation without modification of track layout 

Frauscher axle counting systems can easily integrate with the operator’s existing signaling technology, without modifications to track or layout. Installation is quick, requiring just five minutes per wheel sensor. The sensor is simply clamped to the rail using the Frauscher rail claw, with no drilling required. This is particularly beneficial for brownfield projects, as track access time to complete the work would be limited.

Low maintenance and life cycle costs

Without bonds, wires, and joints to maintain, Frauscher axle counting systems provide low lifecycle costs compared to conventional signaling equipment. The recommended maintenance cycle is just once in two years, involving a simple visual and functional inspection that takes just minutes per sensor. In addition to lower lifetime maintenance costs, the rare need for track access to complete maintenance is invaluable for maintaining uptime on busy transit lines, and for improving worker safety.

Seamless integration and easy cut over to new system

Frauscher axle counting systems can seamlessly integrate with existing CBTC systems. Ethernet protocols, as well as relay and optocoupler outputs, ensure that new and legacy systems can be interfaced with the Frauscher system. Regular testing to be cut over to the CBTC system is not required, since the axle counter acts as an overlay and does not interfere with the legacy system. This is important since every test that is run risks interfering with the proper operation of the vital existing system. 

Track section lengths are not limited in size with Frauscher axle counting systems. In addition, with proper positioning of wheel sensors the dead areas within crossovers that exist with track circuits are eliminated.

Harsh Environmental Conditions and Changing Track and Ballast Conditions


Environmental conditions such as flooding, snow, ice and leaf accumulation can negatively impact the operation of conventional signaling systems. Rusty rails and deteriorated track and ballast conditions can also result in improper operation of track circuit based secondary systems.

Frauscher Benefits

High availability regardless of environmental, track, or ballast conditions

Frauscher wheel sensors are not affected by common environmental and track conditions that can negatively impact the functionality of conventional systems. Pooled water, snow, ice, and leaf accumulation will not result in system errors, nor will rusty rails or other deteriorated track and ballast conditions. 

Trash and metallic debris


Transit systems can experience issues, especially in tunnels, due to pedestrian trash that inadvertently ends up on the tracks. This collected trash can have negative effects on any signaling system. Metallic debris can also cause shunt issues and interfere with proper operation of conventional systems.

Frauscher Benefits

Smart functions further increase availability

Subway tunnels can present a challenging environment when pedestrian refuse unavoidably collects on tracks. Unlike track circuits, changes in shunt resistance will not result in reduced function for Frauscher wheel sensors. 

Metallic objects are also not a problem, as Frauscher axle counters have built in functions that can suppress or correct false activations. These optional functionalities can maintain uptime by increasing fault tolerance of a system. The Supervisor Track Section STS and Counting Head Control CHC functions enable operators to further increase the availability of CBTC secondary systems.

Limited functional scope


Conventional systems can only determine track section occupancy when used as a CBTC secondary system. This limited scope cannot provide train direction, speed, or axle count information, limiting its value as a fallback system. Having this additional information available is important in helping integrators make decisions in Zone Controllers.

Frauscher Benefits

Axle counting, direction and train speed information

Both vital and non-vital additional information can be provided by a CBTC secondary system utilizing Frauscher axle counters. Wheel sensor system occupation status and direction can be provided vitally, while train speed and axle count within a track section are provided non-vitally. Conventional signaling systems only determine whether a track section is clear or occupied, limiting the value of track circuit based CBTC secondary systems.

Limited track section length


A close monitoring of costs is vital to any project requiring a substantial investment, such as a CBTC system. Therefore, it is vital that the secondary system is efficient, but as economical as possible. Due to the limitations in track section length for conventional systems, additional and costly equipment and installation is required to safely cover longer distances, such as two stations that may be several miles apart. This factor may make a conventional secondary system economically infeasible.

Frauscher Benefits

Unlimited track section length 

Track section length is unlimited with a Frauscher axle counting system. Since budgetary factors play an important role in any CBTC project, whether greenfield or brownfield, the secondary system that requires less equipment and less installation time and effort is the preferred choice. The additional equipment required with conventional systems due to their limited track section length, as well as significantly increased installation time and cost for this equipment, makes the Frauscher system the budgetary and time friendly choice. The cost difference for locations where stations are several miles apart is significant.

Traction return current, radiated emissions and electromagnetic interference


Conventional signaling systems can be negatively affected by various occurrences that are common in a transit environment, including electromagnetic interference, traction return currents and radiated emissions from trains. These effects can directly interfere with the operation of the secondary system.

Frauscher Benefits

Highly resistant to EMI interference, radiated emissions and traction return current

Frauscher wheel sensors operate on an inductive principle, and are therefore highly resistant to EMI, radiated emissions and track return current, common issues in a transit system. Reliability of the system is maintained when any of these factors are present. This is not the case with track circuits as fluctuations in any of these levels, resulting from normal rail operations, could disrupt the equipment or cause it to malfunction.

improve your operations


The installation of Communications Based Train Control systems has increased exponentially over the past 20 years. Throughout the world, there are hundreds of mass transit systems, and CBTC is now the norm for greenfield as well as brownfield projects. 

Before 2005, approximately 90% of CBTC secondary systems in North America were track circuit systems. However, since 2005 there has been a consistent change that has moved toward the use of vital axle counting systems. Based on research presented at the June, 2018 American Public Transportation Association Rail Conference by Kenneth Diemunsch, Ted Woods and Roma Mckenzie-Campbe [“How Communications Based Train Control (CBTC) Is Affecting Transit Rail Maintenance”], today over 70% of new CBTC secondary systems utilize axle counters. There are several reasons for this dramatic shift, but most are related to the superior reliability and functional scope of axle counters.

Frauscher axle counting systems provide the functionality, reliability, and low maintenance costs that are ideal for CBTC secondary systems. They also provide ease of installation and the unique capability to seamlessly integrate with existing technology.