This approach is applicable if the Operator has decided to implement fallback mode of operation and the fallback block design is as operationally efficient as their current block design.
This approach will apply two cutover stages before the full CBTC solution is deployed. The first cutover will switch from the legacy conventional system to the conventional system controlled by CBTC (fallback mode). The second cutover would deploy the full CBTC solution (Figure 1).
Once the CBTC solution is deployed, the fallback mode becomes dormant and can be used in the event of a CBTC failure.
Figure 1 - Fallback mode leading to CBTC cutover
Cutover
The purpose of the first cutover is to activate the fallback mode of operation on the CBTC equipment only. The ATS will replace the panel used by the CO, the wayside will mimic the relay logic of the legacy conventional system (the CBTC portion of the wayside software will be dormant) and the VC will be powered off.
From the driver perspective, the system still operates under conventional signalling rules, the only difference being that new signals are installed at different locations along the track (Figure 2).
Figure 2 - Setup before the cutover for fallback leading to CBTC approach
Figure 3 - Day after the first cutover for the fallback leading to CBTC approach
The second cutover involves installing the new ATS, wayside and VC software and any other equipment required for CBTC operation. After the cutover, the VC is powered on and the CBTC functions on the wayside are enabled.
Figure 5 - Day after the second cutover for the fallback mode leading to CBTC approach
This cutover approach can also be deployed in phases (similar to the phase cutover approach) which means implementing a transition zone.
Advantages
If the Operator has an aggressive schedule, the Supplier can deliver the first cutover quickly followed by the CBTC solution at a future date.
CBTC equipment is installed and partially operating.
Operator personnel can adapt to the new equipment while operating under a conventional environment.
Disadvantages
This approach is only possible if the CBTC fallback mode block layout is as efficient as the legacy block layout.
Conclusion
There is no right way or wrong way to cutover a CBTC solution. It all depends on the unique operating environment of the railroad property.
If the Operator has a small 7 km track, the big bang approach will serve their needs while minimizing the disadvantages; a 30 km track would require a phased approach with a design up front for the transition zone.
The Operator must determine the cutover approach they plan to take and feed this requirement into their CBTC specification so the Supplier can cost out the cutover strategy.