Work Zones - The Forgotten CBTC Function

Note: This article is based on chapter 4 of a white paper titled 7 Key CBTC Functions Transit Operators Must Understand which can be downloaded here.

Given that all railroad properties are under constant maintenance, creating a safe corridor for workers at track level, while maintaining service through the work zone is a critical concern for Operators.

In a CBTC application, work zones take on greater importance because the trains are either driverless or operating in an automated mode with a train Operator. If a CBTC train enters an area with workers, the train will not stop; it will continue to move at the same speed. There must be a vital mechanism to inform the CBTC system of workers at track level.

Unfortunately, few Suppliers have a SIL4 work zone implementation. If there is a design, it’s either SIL 2 or SIL 3 with a reliance on operating procedures rather than vitally enforced by the CBTC solution. Operators must have a grasp of their work zone requirements when writing their specification to ensure their workers are protected.

This post will propose a conceptual framework for a vital SIL4 work zone implementation.

Implementing a Work Zone

Setting and clearing a work zone area is a critical aspect of a work zone design and where most Supplier designs fail the SIL4 test; relying on the ATS to set and clear the work zone area is not a SIL4 design because the ATS itself is not designed to support SIL4 functions.

The Operator is forced to rely on communication between the CO at central control and the work crews at track level to set and remove a work zone. A proper design removes the human element (CO communicating with the work crews) and puts the onus on the CBTC system to vitally set the work zone and ensure that work crews have cleared the area before removing the work zone.

An effective design uses work zone tags (different from position beacons) permanently installed on the track. Work zone tags are passive devices with a unique ID. The tags are either placed at regular intervals, such as every 200 meters, or they are placed strategically so that a work zone does not interfere with operations; for instance around interlocking’s (Figure 1).

Figure 11.png

Figure 1 - Work zone tags

A work zone area is defined by two Warning Lights (WL - battery powered SIL4 device with software logic) connected to two work zone tags. When maintenance crews are ready to perform their work at track level, they will connect two warning lights to two work zone tags that contain the work area.

The WL reads the ID of the work zone tag to identify where it is located on the track and transmit the work zone tag ID to the wayside after initiating communication (Figure 2). The WL waits for the wayside to confirm the implementation of the work zone before flashing its warning light.

Note: prior to workers connecting the WL at track level (to implement a work zone), the CO closes the tracks leading to the work zone area. This is done by procedure, which is a separate discussion outside the scope of this paper.


Figure 12.png

Figure 2 - Implementing a work zone using warning lights

The wayside will not implement a work zone if only one WL is installed; it expects to see two WLs connected to two different work zone tags. Once the second WL is connected and transmitting, the wayside will implement the work zone and send a confirmation to both WLs that will start flashing confirming to the maintenance crews that the work zone is implemented vitally (Figure 3). The wayside will also send the work zone status to the ATS to inform the CO.

Figure 13.png

Figure 3 - Flashing WL indicates the work zone has been implemented by the wayside

Once the work zone is implemented, it’s locked. If either or both WLs are removed or fail, the work zone remains in effect. This is to prevent an inadvertent or accidental removal of a work zone, leaving the crew unprotected.

Removal of a work zone requires the maintenance crew to push a work zone release button at one of the two WLs which will cause:

  • Wayside to release the work zone;
  • WL to stop flashing and
  • WL to sound a warning tone indicating the work zone is released

In the remote chance that both WLs fail, the work zone will not be released until a spare WL replaces one of the failed WLs and the work zone release button is pushed.

Work Zone Operations

The purpose of implementing a vital work zone function is to protect workers from a CBTC train and to allow a CBTC train to pass, keeping Service disruption to a minimum. This is accomplished by one of two methods:

  1. Trains arrive at the WL, stop and enter in a non-automated mode; or
  2. Trains enter the work zone at a reduced speed in an automated mode

Note: The Operator must have procedures in place so the work crews and TO are both aware of how a CBTC train will pass through the area. These procedures are outside the scope if this chapter.

In option 1, the wayside will restrict the movement authority (MA) up to the WL. Once the train arrives at the WL and stops, the train operator (if there is a driver) changes to a manual mode with ATP protection and enters the WL under driver control (Figure 4).

The advantage of this option is that the train operator is alert to the activity in front of the train, such as a worker falling onto the track that the VC equipment on the train would not detect.

Figure 14.png

Figure 4 - CBTC Train entering a work zone in manual mode

When the train exits the work zone, the driver places the train in ATO mode and departs automatically.

Under option 2, the wayside will set a speed restriction at the WL and when an automated train arrives it will reduce its speed and enter the work zone. When the train exits, the train resumes normal line speed (Figure 5). 

The advantage of option 2 is that a driverless train can enter the work zone at reduced speed; but this option requires the maintenance crew to be extra vigilant because there is no driver to stop the train if a worker inadvertently falls in front of the train.

Figure 15.png

Figure 5 - CBTC train entering a work zone at reduced speed in ATO mode


A vital SIL4 work zone function is essential for railroads implementing a CBTC solution; because trackside maintenance is a regular routine of a running railroad. Unfortunately, work zone functions offered by most Suppliers demand the Operator to use operating procedures to implement and remove safely.

Two critical aspects of a SIL4 work zone function are:

  • The ability to determine where the work zone starts and ends; and,
  • Confirmation that the work crews have cleared the work zone area before removing the work zone.

The concept proposed here accomplishes this by developing a SIL4 WL and installing work zone tags along the track. The cost of implementing this function is high and therefore Operators must establish the requirements for a work zone function upfront so Suppliers can include the cost of developing this function as part of their overall cost.

Worker safety demands that Operators have a clear picture of their work zone requirements rather than relying on Suppliers to propose a solution dependent on operating procedure.