Expert focus: What is Communication Based Train Control (CBTC) ?

Expert focus: What is Communication Based Train Control (CBTC) ?

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"...track circuits will become relics of a forgotten past only of interest to museum curators and rail enthusiasts."

On a short stretch of track in London, William Robert Sykes tested the first track circuit at Brixton in 1864. In 1872, William Robinson invented the first fail safe track circuit and a method of block occupancy detection was born. 140 years later, block occupancy detection using track circuits (or conventional signalling) is still in use today. Today Consultant and Communication Based Train Control (CBTC) Expert Naeem Ali offers his insights into its characteristics, development and potential. Over the past 25 years the tide is changing as CBTC solutions find their way into traditional track circuit based applications. The primary advantage of a CBTC system is its ability to allow trains to operate safely at much closer headways then is possible in a track circuit based application due to its inherent limitation.

As CBTC becomes the technology of choice, track circuits will become relics of a forgotten past only of interest to museum curators and rail enthusiasts.

But what is CBTC or Communication Based Train Control?

Using the definition from IEEE’s CBTC standard 1474.1, section 4.1 states:

The primary characteristics of a CBTC system include the following:

  1. High resolution train location determination, independent of track circuits.
  2. Continuous, high capacity, bi-directional train to wayside data communications.
  3. Train-borne and wayside processors performing vital functions.

In other words, a CBTC system is able to determine the accurate location of a train, independent of track circuits, using a bi-directional communication link while keeping the system safe.

 CBTC Characteristic #1

The main feature which differentiates a CBTC system from conventional signalling is the ability to determine the location of a train independent of track circuits.

Typically this is done using transponder tags or beacons installed along the track. The tags/beacons provide the train borne unit with a course position. The tachometers installed on the axles provide the fine position.

CBTC_Story_Pic1_ whatisCbtc

As the train crosses tag/beacon B, the train borne unit is aware that it’s located at the 200 meter mark (course position). As the train moves away, the tachometers will count how far the train has moved (fine position). Taking the course and fine position together, the train borne unit will be able to determine that the center of the train is located 247.5m away from the zero reference point.

This is a simplified description (for illustration purposes) of how a CBTC system determines the location of a train.

CBTC Characteristic #2

Once the train is able to accurately determine its location, this information must be relayed to the wayside unit in a timely fashion.

There are various methods to accomplish this. In the past inductive loop was utilized as a communication medium but recently over the past ten years, radio has become the technology of choice for the majority of suppliers. As the technology matures, radio will become the default standard for the rail industry.

For a railroad application, access points are installed along the track. As the train comes within range of an access point, the train borne radio will lock onto its signal and disconnect from the previous access point.


The communication protocols utilized in this medium is usually the standard Ethernet TCP/IP or UDP/IP protocols. This gives the solution flexibility and expandability.

All data (vital and non-vital) is sent through this medium but this link is considered non-vital (TCP/IP and UDP/IP are not considered vital protocols). To maintain safety integrity, end to end vitality must be ensured. This means, the train borne and wayside unit must guarantee the information they receive is not corrupted or stale through various mechanisms (CRC, sequence numbers, Tx ID, Rx ID etc).

CBTC Characteristic #3

It’s not enough that a CBTC system is able to accurately determine the location of a train it also has to protect that train from all types’ failures.

Section 6.1 of 1474.1 lists the vital functions a CBTC system must perform. Reading through this section, it’s quickly apparent that these vital functions can be placed into three categories: collision avoidance, over speed protection and miscellaneous protections.

These three categories are broad in scope and therefore they cannot be covered in a single post (I plan to in future posts) but the basic definition is as follows:

Collision avoidance – Is the ability of the CBTC system to keep trains safely separated from one another and from other obstacles on the guideway.

Over speed protection – Is the ability of the CBTC system to accurately determine the speed of the train and to control the speed within a tight tolerance.

Miscellaneous protection – These are “one of” functions that don’t fit into any generalized category and, in my opinion, are not a fundamental part of a CBTC system. But IEEE has listed them as features that a CBTC system should protect against.

I Thought CBTC Meant Automation?

The primary characteristics defined in section 4.1 provide a basic definition of what a CBTC system is but in recent times CBTC has come to mean much more. When the term CBTC is used, it is commonly defined as an automated driverless system, but nowhere in section 4.1 is there a reference to “driverless” or “automation”.

But IEEE recognizes that there are different CBTC configurations. Section 4.2 of 1474.1 states:

This standard recognizes that different configurations of CBTC are possible, depending on the specific application. For example, a CBTC system may:

  1. Provide ATP functions only, with no ATO or ATS functions.
  2. Provide ATP functions, as well as certain ATO and/or ATS functions, as required to satisfy the operational needs of the specific application.
  3. Be the only train control system in a given application or may be used in conjunction with other auxiliary wayside systems.

At the high end (configuration 3) we have a completely automated CBTC system with ATP (Automatic Train Protection), ATO (Automatic Train Operation) and ATS (Automatic Train Supervision) functionality. At the low end (configuration 1) is the ATP only solution as defined by the primary characteristics in section 4.1 (ATO functional requirements are described in section 6.2 and ATS in section 6.3 of the 1474.1 standard).

The type of configuration a property needs depends on the problem they are trying to solve. If the desire is to increase throughput, then a completely automated system might be needed (Configuration 3). If the desire is to add another layer of safety protection, then an ATP only solution may suffice (Configuration 1).

The point here is a CBTC does not mean “driverless.” At its most basic form, a CBTC system provides automatic protection (ATP) only. More elaborate systems may provide ATO and ATP functionality but it’s not a requirement in order to apply the label “CBTC”.

The adaption of CBTC technologies is spreading far and wide as A) new systems demand a higher throughput and B) current systems try to squeeze more out of their existing infrastructure. As a result, a market has developed where more suppliers are entering the field. 30 years ago only one supplier provided a CBTC solution, today there are three top tier and four second tier suppliers.

The industry has entered a brave new world and the players need to become familiar with the terminology and the various technologies out there so informed decisions can be made.

Note: The opinions expressed in this post are my own and don't necessarily represent Parsons' positions, strategies or opinions.

© 2015 Naeem Ali All Rights Reserved