Transit authorities planning to transition from conventional to CBTC signaling must treat the depot and mainline as a single entity; otherwise the boundary becomes a barrier for launching trains into service. The barrier results from CBTC and conventional signalling speaking different languages; a simplified interface will lose something in translation, preventing a seamless handover of a train from depot to mainline.
Transit agencies planning to deploy a CBTC solution must be mindful that a CBTC solution is effective only when it has control over all aspects that affect mainline operations. The time it takes to launch trains from the depot is also a factor because it compromises the throughput on the mainline. Non-CBTC actors, such as a conventionally signalled depot, hinder a CBTC solution’s ability to control the flow of trains on the mainline, reducing the advantages CBTC was meant to introduce.
Implementing a CBTC solution on the mainline and leaving the depot conventionally signalled is a mistake.
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).
The positional accuracy of a GPS depends on the environment it’s operating in; if six satellites are in the sky on a clear sunny day in the countryside, the positional accuracy will be high. But if there is a thunderstorm in the area with only 3 satellites in the middle of a metropolis, the positional accuracy diminishes. Pinpoint accuracy requires ideal conditions.
Similarly, a CBTC train’s ability to report an accurate position depends on the design of the train and the track it’s running on. Train parameters and track characteristics must be understood and controlled to reduce positional uncertainty; otherwise the train cannot be tracked nor can it be protected.
During my teenage years, my driving instructor taught me the three second rule; the minimum separation between my car and the car in front had to be three seconds. He drilled this rule into my head, but a rule for cars does not translate well when driving a fifty ton train at 60 km/h around a blind curve. Something a little more sophisticated is required.
The equivalent guideline in the conventional signalling world is the “one block separation” rule; the distance separating two trains must be one block. This 150 year old practice ensures that if a train is tripped there is enough distance to stop the train.
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.
Tracking trains using track circuits has been the conventional wisdom for the past 150 years. No single invention in the history of rail has contributed more towards safety then the track circuit. This simple invention is the foundation for block signalling and the primary method of tracking trains through a mass transit system.
However, over the past 30 years CBTC technologies have rendered this foundational component obsolete. Therefore, how does a CBTC system track trains without track circuits?
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.