Note: This article is based on chapter 1 of my white paper titled 7 Key CBTC Functions Transit Operators Must Understand. Download it here.
Train recovery is a critical function because it defines how the Operator will recover a failed train under a worst-case failure; defined as a VC unable to communicate the train’s position to the Wayside (the Wayside cannot track the train). If the CBTC design can handle the worst-case scenario, then all other train recovery scenarios are taken care of automatically.
A stranded train due to communication failure is a rare event due to the built-in redundancy all CBTC solutions provide: redundant network design, redundant radios on the trains, overlapping radio coverage and hot standby VCs; nonetheless the CBTC solution must have a design in place to recover from this rare event.
In this scenario, a communicating train (CV) is unable to transmit its position to the Wayside (radio failed). The non-communicating train (NCV) will brake to a stop and Service behind the train will halt (Figure 1). The Operator must decide how to rescue this train from the mainline and allow Service to continue.
Note: the discussion from this point forward assumes a train operator (TO) walked to the NCV or a TO was already on the train when communication failed (some systems demand a TO be on the train even though it is an automated system).
Figure 1 - Worst case train failure
If the Train Operator (TO) switches to manual mode and moves the train, it becomes a ghost train (see Figure 2) because it’s not reporting its position and the wayside is unaware the train is moving.
This invokes CBTC rule number one, NCV’s are not permitted to move without protection.
Figure 2 - Ghost Train
Challenge for the Operator: a train packed with commuters is not able to re-establish communication with the wayside and cannot move in manual mode. The Operator must decide how the CBTC design will recover this failed train.
The Operator has three design options available:
- Train Protection Reservation (TPR) – Create a safe corridor between two points on the track to allow a non-communicating vehicle (NCV) to travel safely within.
- Train coupling – A communicating vehicle (CV) tows a non-communicating vehicle (NCV).
- Fallback mode of operation – Secondary detection devices are utilized to track the non-communicating vehicle.
Each option is more complicated and costly than the last, but the operating environment ultimately determines which option is applicable to the Operator.
Train Protection Reservation (TPR)
Note: This section is based on IEEE 1474.3 section 6.1.4
The TPR is a basic building block for any CBTC solution. It is created at the request of the CO to isolate a section of track to permit a failed train to travel safely within. The TPR prevents switches from moving and automatic trains from entering and/or operating inside the TPR (Figure 3).
Figure 3 - TPR
Once the TPR is locked down, the CO will give the train operator (TO) permission to move the NCV within the TPR. When the train reaches its final destination, the TPR can be removed either by manual procedure or a design that verifies the TPR is cleared of all obstructions.
The advantage of the TPR function is the simplicity of its design. The disadvantage is if the final destination is far off, the TPR will cover a large section of track, which means Service is impacted until the train reaches its destination. To counter this, small TPRs can be set until the train reaches its final destination, such as station to station or station to switch.
The TPR is a basic protection mechanism that allows an NCV to safely travel from its current location to the final destination.
Next: Part 2 of this post will discuss the train coupling and fallback mode of operations recovery methods.