Signalling SystemsIntroduction IntroductionA variety of signalling and train control solutions are available for modern urban rail transit systems depending on the specific operational requirements and the rail transit technology selected. The simplest type of urban rail transit system is the streetcar or tram where vehicles run on steel rails sharing road space with other road users. These systems typically operate at wide headways (the safe spacing between consecutive vehicles), but can operate at reduced headways of around two minutes. This reduction, however, leads to bunching of vehicles and a less reliable service. For streetcar systems, the normal operational mode is “line-of-sight” or “drive-on-sight” and can be supplemented by signalling control. Track switches are either controlled by the driver or the signalling system, through some form of train-to-track control centre communication. A passenger information system, an automatic vehicle location (AVL) system plus some level of centralised traffic supervision may be the only signalling equipment in a streetcar network. For more sophisticated urban rail transit systems either a full or partial Automatic Train Protection (ATP) system becomes mandatory to provide acceptable levels of passenger safety and is a suitable mitigation against the deficiencies of line of sight operation that can ensure that optimal headways and higher speeds are achieved without compromising system safety. One example of an urban rail transit system that may require partial ATP is LRT (Light Rail Transit) systems, which fall between a streetcar and a full “heavy rail”/metro system. In LRT systems, full ATP is typically required in tunnel sections and at main junctions within the network. ATP functions may also be extended to other sections of the network, for example for speed monitoring and/or for the prevention of signals being passed when displaying a STOP aspect, typically at road crossings. ATP operation allows for continuous operation at headways of two to three minutes while maintaining service reliability. LRT operations also communicate and control priority to ITS and road traffic signals and ensure the LRT vehicle has right of way through any junctions, with the exception of emergency vehicles which require the fitment of transponders to override the priority system. In the event of signalling failure the road signal junction controller has a key operated push button to allow the train driver to manually operate the junction signals and allow the vehicle clear passage. Metro, or “heavy rail” systems, typically operates at two to three minute headways in peak periods depending on the line and rolling stock designs, allowing for up to 30 trains per hour per direction. With more sophisticated signalling, headways of 90 seconds are achievable. Most metros operate on a 100% dedicated infrastructure, generally in tunnels and/or overhead structures and require a very high operational reliability. In addition to full ATP, many metros also include ATO (Automatic Train Operation). Automatic People Movers (APM) are most often being used for feeder links such as at airport areas. APMs are always driverless and hence require both ATP and ATO, with all functions that would normally be performed by a train driver being fully automated. While APMs are often required to operate with short headways, the systems are usually quite short in length, and the peak capacity is rarely greater than 2,000 to 3,000 pphpd. There is also a recent clear trend to implement driverless operation on metros, especially for newly built lines. Existing metro lines that are being re-equipped are also beginning to move to driverless operation with full ATO. Mixed mode operation, both driver-less and with driver-controlled trains on the same line, also exist. Signalling systemsAt its most basic, the signalling system safely controls the movement of many trains, and allows the Control Centre to monitor where the trains are and how the system is running. There are a variety of signalling and train control solutions that are available for modern urban rail transit systems depending on the specific operational requirements and the rail transit technology selected: Line-of-sight or drive-on-sight is the most basic. The operator on the train watches for the train ahead of him, and operates based on a set of rules. While some systems may or may not be signalled, there may be voice and/or data communication with the Control Centre. Line of sight operational headways have limitations related to driver visibility and for safety reasons, track curvature, gradients and infrastructure impact maximum operational speeds that can be achieved.
Line of sight, wayside signals, cab signals and CBTC are all in use today. Wayside and cab signals are mature technologies which offer good, proven control, but do not allow for automation. Varying degrees of CBTC will allow the system to operate without on-board operators, but increase sophistication, requirements for access control, and require a larger and more complex control centre. While many of the operating staff are removed from the vehicles, surplus staff can be reallocated to customer service functions and technical response functions in the event of system failure. If CBTC is accompanied by ATO and depending on public access to the transit infrastructure then some re-allocation of staff may be required to maximise public safety and station and vehicle security. Line of sight (Drive on sight)Is the most basic form of vehicle control and can be supplemented with wayside or cab signalling. Line of sight operation is very dependent upon driver training and the establishment of driving rules to increase passenger safety level. One of its major benefits is that in the event of signalling or subsystem failure. The vehicles can still be driven under ‘rule’ with the control centre coordinating the driver’s movements by radio or auxiliary communication system. Wayside signallingA very mature and simple technology that relies on the illumination of aspects to indicate to the driver the state of the track ahead of the vehicle. The position of the vehicle is detected by a series of track circuits in the rail, which are bridged or short circuited indicating its position. The detection system is therefore very course and can be prone to detection failure in the event of corrosion on the rail. However, the system is designed to fail-safe so that the signalling system will default to a red aspect or ‘stop’ in the event of system failure. The signalling system can be implemented using electronic relay or solid-state technology, with the latter becoming more prevalent. The design and construction techniques of such a train control system are very rigorous and well established and will result in high reliability and safety. The disadvantage of wayside signalling is that considerable electrical and mechanical construction is required to implement this and as this is a safety critical system specialists must maintain it. However the system can be very basic and easy to implement at lower cost. The techniques are well understood around the world and as such specialist knowledge is obtainable. Wayside signalling operated by relays tends to be fixed block (i.e a number of track circuits denote the fixed block length). This method of signalling can be limiting in terms of future expansion. Cab SignallingIs accomplished by the transfer of vital data from wayside radio antenna or balise in the track bed. Effectively the wayside signal head and aspects are positioned in the cab area. Visually this approach is better for the driver as he does not have to deal with varying weather conditions or poor sighting of the signal heads at the wayside. Another major benefit is in the upgrade or renewal of track as the wayside signals will require decommissioning and re-sitting on the track. With in-cab signalling, it is only the radio antenna and data that require alteration. It should be noted that the as the data from the antenna is vital, this is normally encoded with error detection and correction and that there are redundant communication channels to ensure reliability of data. Another benefit is that the cab signalling can be developed away from the vehicle as part of the systems acceptance test. CBTC SignallingCBTC although relatively new in comparison to conventional relay based signalling, is in a fairly advanced state of execution around the world. It should be noted that a number of these systems tend to be mainline or commuter routes which operate over large distances. Its major benefit is that it can be deployed using wireless transmission methods and example being GSM cellular networks. CBTC deployment has tended to be more conservative with less automatic controls being passed over wireless or telecommunications links, however the benefits of cellular communications or mesh networks provide redundancy and fast data transfer and the ability to share costs or lease cellular lines from telecommunications companies. The use of wireless technology to implement the signalling system will greatly assist commissioning as this can be implemented in parallel with the construction of the right of way. A major disadvantage with the implementation of wireless or radio communications for Ottawa will be its poor coverage in the tunnel or enclosed areas; however ‘leaky feeders’ can be implemented in tunnel areas to repeat signals in the bores. As CBTC is less mature, there are fewer experts available that have a combined knowledge of telecommunications, safety techniques and railway signalling principles.  The initial applications of CBTC in North America were the Scarborough RT line in Toronto, the SkyTrain in Vancouver, and the Downtown People Mover in Detroit, all in the mid 1980’s. Since then, CBTC systems have been widely deployed around the world – specifically in Europe and Asia – and are available from multiple suppliers. To date, CBTC systems have been adopted primarily in applications where short headways and higher levels of automation (including fully driverless) are key operational requirements, and as a consequence these systems have been relatively sophisticated with complex functionality and with a large number of application-specific external interfaces. There are three fundamental characteristics of CBTC:
Train Location Determination: Knowledge of the location of all trains within a transit network is a fundamental requirement for any level of train control or operations management. With CBTC technology, train location determination is no longer constrained by fixed-block track circuits and track-based equipment, but can be determined to any required precision, as required to support and optimise the transit system operations. CBTC technology can provide very high precision train location determination for those applications that require the highest possible train throughput on a given infrastructure. But for those applications where short headways are not critical, the precision in train location determination can be relaxed, and the associated system implementation simplified. CBTC systems typically utilize train-borne equipment to establish train location, with wayside devices limited to periodic track-mounted tags and/or GPS satellites. However, if the train location reports are to be used for train protection purposes (collision/derailment prevention/avoidance) then the train location determination function must also be designed to meet applicable safety criteria. Data Communications: The ability to communicate control and status information to and from a moving train is also a fundamental requirement for any level of train control/train management and the data communications system must have sufficient bandwidth, appropriate message latency, and acceptable levels of message security, to meet the operational needs of the specific application. For example, when short headway/high capacity operations are critical, message latency must be very short, and reliability of message delivery must be very high. However, these requirements can be relaxed, and the data communication system simplified, if the specific application does not require short headway applications (i.e. less than 2 minutes). CBTC systems today typically use radio as the data communication medium, but CBTC is not constrained to any particular operating frequency, or any specific data communications protocol. The data communications system, in addition to supporting train control functions, can also support a wide range of passenger information, train maintenance data management, Wi-Fi, CCTV and other functions. Special requirements are needed for operation of radio systems within tunnel areas. Data Processors: With CBTC technology, the control and status information communicated to and from a moving train can be used for a wide range of train control and train management functions, up to and including fully automatic, unattended train operations. For manually-operated trains, the complexity of the train-borne equipment, interfaces to other train subsystems, and sophistication of associated wayside equipment, can of course be significantly simplified and limited solely to basic train protection functions, for example, with advisory commands to the train operator related to train dispatching and train regulation. The basic train protection functions include:
Typically, these hazards are addressed through
ConclusionA signalling system is mandatory for a transit system to provide safe and reliable operation and maximise the number of vehicles on the line. Conventional relay based or solid state signalling provides a proven and mature method of controlling trains with minimal risk but with the penalty of some inflexibility later if upgrades or a change to the line occurs. To all intents and purposes the passenger is relatively unaffected by the choice of signalling system except in times of degraded operation or failure or high capacity. With sufficient capacity designed into the system to meet demand in later years there is little to choose from, however subsequent additions to the vehicle fleet would have to incorporate the ancillary equipment associated with the existing signalling system. One of the bigger factors in signalling systems is that they become obsolescent due to component manufacturers not supporting the equipment for the lifespan of vehicle. As such although the design life of the overall system is aimed to be 75 -100 years, in this timeframe it is likely that there will be at least 3 or 4 signalling upgrades. Increased automation brings about higher levels of complexity, which can compromise public safety, and reliability of system operation. The increased complexity requires additional workload to commission and certify the system for safe operation and the operational team are required to be ‘highly’ safety focussed to regularly maintain the system. In cab signalling reduces the wayside signalling infrastructure and provides a more flexible approach to upgrading the line at future date at the expense of some more complexity. The in cab signalling is not affected by climatic conditions and therefore may be more suited to the Ottawa environment. CBTC signalling offers greater flexibility and future proofing but may be more considerably complex than a mass transit system requires for short distances. This approach carries some risk of implementation but could be designed in as part of future upgrades. CBTC should not be viewed simply as a “signalling system” but rather as an “enabling technology” to permit a step-change improvement in the level of service being offered to transit passengers. The concept of CBTC is therefore not restricted to high capacity, automated, heavy rail/subway systems, but can also offer benefits to a wide range of other transit modes, including Light Rail Transit (LRT). The trends in adopting CBTC indicate that it:
|
CON044293