CBTC Explained - An Animated Course
Sometimes a quick discussion can solve the trickiest problem
Urban population densities are increasing and transit operators are demanding more from their transit infrastructure to reduce headways and increase throughput.
Unfortunately, technology and signalling philosophies developed in the 19th and 20th centuries are not meeting that challenge.
As a result, rail automation is gaining steam. Over the past 30 years, transit operators are shifting to semi or fully automated signalling systems.
The benefits of automation are clear: reduce human error, increase safety, reduce maintenance and increase operational performance but, this comes with a cost in terms of complexity and price tag.
Therefore, how does a transit operator decide how much automation is enough automation for their property?
Experienced CBTC Transit Operators keep a laser focus on their diagnostic design because the time it takes for the Operator to identify a problem, localize the problem and fix it is determined by the diagnostics capabilities of the CBTC solution.
Sophisticated diagnostics enable the Operator to recovery from failure quickly whereas rudimentary diagnostics delay recovery while commuters are stuck on the track.
Given that all railroad properties are under constant maintenance, creating a safe corridor for workers at track level, while maintaining service through the work zone is a critical concern for Operators.
In a CBTC application, work zones take on greater importance because the trains are either driverless or operating in an automated mode with a train Operator. If a CBTC train enters an area with workers, the train will not stop; it will continue to move at the same speed. There must be a vital mechanism to inform the CBTC system of workers at track level.
Operationally critical functions must be understood when deploying a CBTC solution. These functions define how a railroad operates once the solution is deployed and if neglected the Operator can expect service disruptions, longer recovery times and irate commuters. Laser-focus on the CBTC solution’s operational functions will ensure that the operational requirements of the Operator are satisfied.
Fallback mode in a CBTC application, is a legitimate mode of operation but avoid it when possible. The cost of implementing a fallback mode will outweigh the marginal benefits that fallback will provide: increased complexity, increased maintenance cost and up to 30% increase in capital costs. Yet some operators handcuff their solution by imposing a fallback mode requirement without understanding the need.
The operating environment ultimately determines if fallback mode is required and which of the multiple options is selected. The operator must take a methodical approach when evaluating the need for fallback because the consequence of making the wrong decision are costly.
The safety record for conventional signaling is beyond doubt; 140 years of improvements, handed down by thousands of engineers has ensured the safety of our urban transit infrastructure.
However, transit authorities are switching to CBTC in increasing numbers and the primary motivation is due to the operational superiority, not safety, of a CBTC solution over a conventional one.
CBTC shines because it pushes the operational envelope (shorter headways) while maintaining the proper level of safety. Whereas a conventional system is handcuffed operationally due to its inherent limitations (fixed block signalling philosophy).
Traditional fixed block signalling grew out of the invention of the track circuit which gave the signalling community its first failsafe method of detecting the presence of a train. This method has served the signalling community well for the past 150 years but as the population of major urban centers grow, the demands on transit operators also grow.
In the past, signal engineers included a margin in their fixed block design which allowed the operator to increase the frequency of service, but many cities have exhausted this margin or are fast approaching it.
As a result the operators have two choices; build more subway lines or squeeze more out of their existing infrastructure by adapting new technologies; such as moving block based on CBTC technologies.
On a short stretch of track in London, William Robert Sykes tested the first track circuit at Brixton in 1864. In 1872, William Robinson invented the first fail safe track circuit and a method of block occupancy detection was born. 140 years later, block occupancy detection using track circuits (or conventional signalling) is still in use today.
Over the past 25 years the tide is changing as CBTC solutions find their way into traditional track circuit based applications. The primary advantage of a CBTC system is its ability to allow trains to operate safely at much closer headways then is possible in a track circuit based application due to its inherent limitation.
The Automatic Train Supervision (ATS) system plays a vital role in monitoring the performance of Communication-Based Train Control (CBTC) signaling systems during revenue operations. Despite its significance, Transit Operators often neglect to properly define the ATS user interface requirements in their specifications, resulting in a CBTC supplier delivering a cumbersome and tedious user experience that is unacceptable for such a crucial subsystem.