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.
What is Fallback Mode of Operation?
Fallback mode is an auxiliary method of detecting trains using conventional means, it is not the primary method. Dr. Alan Rumsey defines fallback (secondary train control system) as follows:
“… a secondary train control system is defined as signaling equipment that, when integrated with the primary CBTC system, provides a level of automatic train protection (ATP) functionality for trains either:
- Not equipped with train-borne CBTC equipment, and/or
- Operating with partially or totally inoperative train-borne CBTC equipment, and/or
- Operating within an area of track with partially or totally inoperative wayside CBTC equipment.
A secondary train control system is not a complete/stand-alone signaling/train control system, but rather auxiliary equipment to provide partial ATP functionality for the movement of non-CBTC-equipped trains and/or the movement of CBTC-equipped trains in the event of certain CBTC system failures.”
In other words, fallback mode of operation is the ability to track trains with track circuits or axle counters if the primary CBTC signaling system is inoperable, not available or non-CBTC equipped trains (NCT) enter the system.
How does it Work?
Of the different types of fallback mode, the favoured option is mixed mode of operation (option 3 below) because of its robustness to handle the needs of any transit operator. But less popular methods such as fallback mode or hybrid fallback mode are also used and described below.
Option 1 - Fallback mode (conventional signalling) – This mode runs in isolation of the CBTC system (see figure 1 below). When the CBTC system is operating, the fallback mode is not. If there is a CBTC system failure (such as a radio network problem), the entire system is switched to fallback mode and the trains move according to conventional signaling rules. Once the problem is fixed, the system will switch back to CBTC mode. This mode is useful during the initial deployment and transition from conventional signalling to CBTC.
Option 2 - Hybrid fallback mode – This mode confines fallback mode to a small section of track (see figure 2 below). If there is a localized radio network failure preventing CBTC operations in that area, fallback mode would be applied only to this section of track. Trains would approach in CBTC mode and stop just short of the fallback area, switch to manual mode and proceed using conventional signaling rules. Once the train clears the fallback area, it would switch back to CBTC mode and continue to its destination.
Option 3 - Mixed mode operation – CBTC equipped and non-equipped trains operate on the same track at the same time during revenue operations (see figure 3 below). The artificial intelligence behind the CBTC system is aware of the conventional fixed block occupancies and treats them as an obstruction; when a CBTC train approaches, it will stop a safety distance away.
In the above example, all trains (except blue) will follow CBTC rules, which keep a minimum safety distance with the train or obstruction in front.
Since the blue train is not CBTC equipped, it will be tracked by track circuits and will therefore follow conventional signaling rules and keep a minimum one block separation from the train in front and behind.
Each train will behave as follows:
- The mustard train will maintain the absolute minimum safety distance from the black train in the same block (CBTC rules).
- Since block 3 is occupied by a non-equipped train (blue train), the entire block will be treated as an obstruction and a block separation will be maintained in front and behind the train. The black train will stop a minimum safety distance from signal B. Although, the black train could travel up to signal C, the CBTC system would not be able to determine if block 2 is occupied by the black or blue train.
- The blue train will stop at signal D because it must maintain a one block separation from block 5 which is occupied (conventional rules), even though the green train in block 5 is a CBTC train.
The conventional signalling component is fully integrated into the CBTC solution allowing the two modes to exist at the same time. This means:
- If there is a system-wide radio failure, trains can operate using conventional signaling rules.
- If there is a localized radio problem, the trains can move through the affected area using conventional rules and CBTC rules when outside of the affect area.
- If there is a non-equipped maintenance train that needs to enter the system to conduct emergency maintenance, the maintenance vehicle can move using conventional rules and all other trains will use CBTC rules.
Mixed mode of operation is the most flexible option because of its ability to handle all scenarios.
Is it For Me?
The benefits of CBTC operation have been proven in revenue service operation but the perception remains that a CBTC system must be supplemented by a fallback mode of operation. To determine if fallback mode is needed, the operating environment must be tested against established criteria:
Criterion 1 - Is the railroad property an open or closed system?
In an open system, CBTC equipped trains coexist with non-equipped trains. This occurs on shared tracks where CBTC equipped commuter trains operate with non-equipped freight trains on the same section of track. The commuter train will operate using CBTC rules, such as moving block, and the freight train will operate using fixed block conventional rules.
The two signaling methodologies must coexist; the CBTC and non CBTC train must be able to detect each other and maintain the proper safety distance according to their signaling methodology.
In a closed system, only CBTC equipped trains operate; non-equipped trains never enter the system.
Criterion 2 - Are maintenance trains equipped, if not, can operational procedures be used?
Transit operators may decide not to equip their maintenance vehicles due to the cost of the fitment program or other technical problems. In this scenario the operators have two options, either implement a fallback mode or operate the maintenance vehicles using operating procedures; the maintenance vehicle would operate on the track undetected and operating procedures would protect the maintenance vehicle.
Operating procedures are a viable option but this depends on the operating environment. If the transit property has 50 to 60 maintenance vehicles operating every night, procedures will be difficult to implement for each vehicle. But for a small operator with 2 or 3 maintenance vehicles, operating procedures are adequate.
Criterion 3 - What kind of failure recovery is required?
If the operator has a closed system and the maintenance trains are equipped with CBTC equipment, the next question is what kind of recovery is desired in the event of CBTC equipment failing on the train?
In this scenario, the operational requirement is to safely remove the train from the mainline. However, if the CBTC equipment has failed, the system cannot track or protect the train and removing the train becomes a problem (see figure 3 below).
In the scenario above, the following recovery options are available:
- Implement an operating procedure where the section of track from the failed train to the next station or yard is closed to train traffic; the CBTC system can include functions to support this. The failed train can safely travel through this corridor but it would be an operational nightmare if the distance to the yard is far off.
- A functioning CBTC equipped train can couple to the failed train and tow it back to the yard. The functioning train would extend its envelope to include the failed train to provide protection.
- A fallback mode will allow the failed train to travel using conventional signaling rules to its destination.
In a small system (8 to 10 km), recovery Option 1 would be sufficient but in a large system with hundreds of kilometers of track, a coupling or fallback mode may be required (Options 2 and 3).
Unless there is an operational requirement, based on the criteria described above, fallback mode should be avoided. The cost of implementing a CBTC solution with fallback will outweigh the marginal benefits the fallback mode would provide; an order of magnitude of complexity is added to the overall solution and the extra equipment will reduce reliability and increase maintenance.
Instead, a focus should be placed on the failure recovery functions that can be implemented as part of the CBTC solution such that the operator does not need to rely on operating procedures.
A thorough analysis must be conducted to determine if fallback mode is absolutely needed because the consequences of a wrong decision will remain with the operator for the life of the CBTC solution.
Rumsey, Alan. “So who really needs a "Fall-Back" Signaling System with Communications Based Train Control?” APTA 12 Dec. 2012.