Note: This article is based on the introduction of a white paper title 7 Key CBTC Functions Transit Operators Must Understand which can be downloaded here.
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.
Functions that define a CBTC solution are split into two broad categories; core functions and non-core functions (Figure 1). Core functions handle the basic Automatic Train Protection (ATP), Automatic Train Operation (ATO) and Automatic Train Supervision (ATS) functionality such as positioning, train tracking, propulsion & braking control, routing, movement authority, interlocking, regulation, scheduling and communications.
Figure 1 - Core CBTC functions vs non-core functions
uppliers generally refuse to change core functions from project to project due to complexity and cost (there are exceptions). Therefore, Operators have no choice but to accept these functions as is, otherwise they should seek another CBTC solution.
However, Operators have influence over the design of non-core functions because the Operator’s personnel and the riding public interact directly with these functions and the Suppliers recognize this. Non-core functions must meet the Operator’s unique operational needs otherwise the inadequate functions will hamper the Operator’s ability to deliver satisfactory Service.
For example, a Supplier developed a work zone protection function on a previous project for Operator A and would like to implement this function with no changes on Operator B’s property. This function may have been applicable to Operator A but due to the unique operating environment of Operator B, this function must be modified to fit. The Supplier will resist but the Operator must push.
Ignoring non-core functions means blindly accepting the Suppliers proposed solution. Understanding the operating environment is crucial to understanding the everyday scenarios encountered when running Service such as Service build up, diagnostics & maintenance, recovery from Service affecting faults or passenger information and announcement systems. Only then can the Operator create a design framework for a CBTC solution that the Supplier must follow.
Of the numerous non-core CBTC functions, seven are key because of the multiple implementation options available or their importance is not appreciated by most Operators. They are as follows:
Key Function #1: Train recovery – In a signalling system with no secondary train detection (track circuits or axle counters), this is a critical function when a train is unable to communicate with the wayside (considered the worst-case failure in this paper). There are three train recovery options and each one increasing in complexity and cost; the Operator must select an option based on their operating environment.
Key Function #2: Work zone protection – Creating a safe corridor for workers at track level while maintaining Service level is a critical concern for all Operators. In a CBTC application, the work zone function takes on greater importance because the trains are either driverless or operating in an automated mode. A vital SIL4 (Safety Integrity Level 4) design is required to inform the CBTC system of workers at track level, while maintaining Service.
Key Function #3: Equipping Work Cars with CBTC equipment - Equipping work cars with CBTC equipment is not a function but a decision and operationally, a critical one. Work cars must coexist with passenger carrying trains either by equipping work cars to follow the same rules as passenger carrying trains (consistency) or by applying special rules to unequipped work cars.
Operators with a fleet of 60 or 70 work cars may opt for rule book consistency and equip work cars whereas small Operators may opt for special rules and operate with unequipped work cars. Every railroad property is unique and how they operate their work cars in a CBTC environment is no different.
Key Function #4: Diagnostics – Effective diagnostics allow a CBTC system to localize and pinpoint problems, permitting the Operator to quickly recover from Service affecting faults.
A proper diagnostic architecture has three levels and each level increases the resolution of the problem. All suppliers utilize a level 1 architecture, most implement a rudimentary level 2 but no Supplier has successfully implemented a level 3 diagnostics architecture.
Key Function #5: Fallback mode – Fallback mode is a mechanism to track trains using secondary detection devices such as track circuits or axle counters. It is a legitimate mode of operation, but avoid it when possible. The cost of implementing a fallback mode will outweigh the marginal benefits this function provides. The Operator must take a methodical approach when evaluating the need for fallback because the consequence of making the wrong decision is costly.
Key Function #6: Launching trains - 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. 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 a factor.
Key Function #7: Cutover strategy –The Operator must select the right cutover strategy to transition from conventional signalling to CBTC with minimum impact to Service. The Operator must have an approach in mind so the Supplier can design a solution that supports the strategy.
Note: Each key function is described in my white paper. The download link is provided below.
CBTC Suppliers have developed very good core CBTC functionality but the non-core functions are generally lacking. Unfortunately, non-core functions are critical to operations and therefore the Operator must have an operational focus within a CBTC context when deploying a CBTC solution; the Operator must translate their existing operational philosophy, procedures or methods into a CBTC design framework the Supplier can understand and implement.
If the operator has a grasp of the seven key non-core CBTC functions described above, the Operator will have greater control over CBTC solution deployed on their property.