To further explain the types of Computer Based Train Control, commonly called CBTC, it is helpful to examine real-world systems, deployment trends, and the communication technologies being used and developed.
The specific type of CBTC selected for a railway depends on its operating environment, infrastructure, capacity requirements, safety standards, and desired level of automation.
Most currently operating CBTC systems use a centralized architecture.
In a centralized system, trains continuously send their location, speed, direction, and operating status to central control equipment.
The central control system calculates train separation, Movement Authorities, speed profiles, and other operating commands and sends this information back to the trains.
Examples of major centralized CBTC systems include Siemens Trainguard MT, Thales SelTrac, and CITYFLO, which was originally developed by Bombardier and is now part of Alstom.
These systems are used in many urban rail and metro systems throughout the world.
Some newer CBTC systems are exploring hybrid or more distributed architectures.
These systems may place additional processing and decision-making capability onboard the trains while still using a central control system for overall coordination and safety management.
A hybrid approach may improve scalability because more trains can be added without placing all processing demands on one central system.
It may also improve resilience because some train-control functions can continue even if communication with a central component is temporarily reduced or interrupted.
Many urban transit systems begin CBTC operation at Grade of Automation 2, commonly called GoA 2 or Semi-Automatic Train Operation.
At GoA 2, the CBTC and Automatic Train Operation systems can control train acceleration, speed, braking, and accurate station stopping.
A driver is still present and is normally responsible for door operation, emergency response, and manual operation during degraded conditions.
GoA 2 provides many of the capacity and efficiency benefits of CBTC while retaining an onboard operator.
There is an increasing movement toward higher levels of automation, especially on new metro lines and major signaling upgrades.
GoA 3 provides Driverless Train Operation, while GoA 4 provides fully Unattended Train Operation without onboard operating staff during normal service.
Higher automation can improve operating consistency, reduce headways, support more frequent train service, and reduce some operating costs.
Examples of highly automated urban railway systems include Paris Metro Line 14, Singapore's North East Line, and the Dubai Metro Red and Green Lines.
Implementing GoA 3 or GoA 4 on mainline railways is generally more difficult than implementing it on a closed urban metro system.
Mainline railways may include passenger trains, freight trains, maintenance vehicles, highway grade crossings, longer braking distances, and trains with different operating characteristics.
Mainline systems may also require more advanced obstacle detection, emergency response procedures, and coordination between several train control systems.
Because of these challenges, fully driverless mainline train operation is still less common than fully automated urban metro operation.
Different CBTC suppliers use different radio frequency bands, communication protocols, and network designs.
Some systems use dedicated licensed radio frequencies, while others use unlicensed frequencies such as those commonly associated with Wi-Fi.
Licensed frequencies may provide better control over interference but can be more expensive and may require government approval.
Unlicensed frequencies may be easier and less expensive to deploy, but they may experience more interference from other wireless equipment.
The selected communication technology must provide sufficient coverage, bandwidth, reliability, security, and resistance to interference.
Research and development are continuing into the use of higher-frequency millimeter-wave communication for railway systems.
Millimeter-wave technology may provide greater data capacity and support more advanced train-control, monitoring, and passenger-information functions.
However, higher-frequency signals may have shorter operating ranges and may be more easily blocked by structures, curves, tunnels, weather, and other physical obstructions.
Some developing CBTC systems are exploring the use of public cellular networks such as LTE and 5G.
Using a public cellular network may reduce the amount of dedicated wayside communication equipment that a railway must install and maintain.
Public networks may also provide broad coverage, high data capacity, and easier technology upgrades.
However, railway operators must carefully evaluate network availability, communication delays, priority access, cybersecurity, coverage, and reliability before using public networks for safety-critical train control.
There is no single CBTC design that is suitable for every railway.
A system may combine centralized control, onboard processing, a specific Grade of Automation, and several communication technologies.
Railway operators must consider system capacity, train types, track layout, braking distances, operating rules, cybersecurity, maintenance requirements, and available communication infrastructure.
The final CBTC design must be matched to the railway's specific operational and safety requirements.