SelTrac is a digital railway signalling technology used to automatically control the movements of rail vehicles. It was the first fully automatic moving-block signalling system to be commercially implemented.
What is now branded as SelTrac was originally developed in the 1970s by Standard Elektrik Lorenz (the "SEL" in the name) of Germany for the Krauss-Maffei Transurban, an automated guideway transit system proposed for the GO-Urban network in the Greater Toronto Area in Canada. Although the GO-Urban project failed, the Transurban efforts were taken over by an Ontario consortium led by the Urban Transportation Development Corporation (UTDC), and adapted to become its Intermediate Capacity Transit System (ICTS). This technology was first used on the SkyTrain network in Vancouver, British Columbia and the Scarborough RT in Toronto, Ontario.
SelTrac was primarily sold and developed by Alcatel, through a subsidiary. SelTrac is now sold by Thales from their Canadian unit, after it purchased many of Alcatel's non-telephony assets. New versions were made for different markets, and today SelTrac is used for rolling stock control around the world and is branded as a communications-based train control (CBTC) system.
The original SelTrac system was based on inductive loops that provided a communications channel as well as positioning information. Normally an inductive loop is used solely as a communications system, with electromagnets on the vehicles or stations inducing currents in the loop that can be read at a distant location. In the case of SelTrac, the central computer sent data to the vehicles at 1200 bit/s on a 36 kHz carrier while the vehicles had 600 bit/s on a 56 kHz carrier. Separate pair of antennas is used for transmission and another pair is used for reception.
The two-wire inductive loop system was prone to vandalism and, in newer versions of SelTrac, the control signal is transmitted inside the running rails at radio frequency using IEEE 802.11 (WiFi) access points.
SelTrac uses the twisted-loop concept developed by Siemens in the 1950s. SelTrac's loops cross every 25 meters to form lozenge-shaped areas. The communications system on the vehicles can detect a phase change in the signal caused by these crossover points, allowing them to place themselves within a single one of these sub-loops. Position within the loop can be further measured by counting axle revolutions. The vehicles broadcast this position information into the loops, along with IDs, speed, direction and other data.
This system is not accurate enough for positioning within a station. Further accuracy is provided by "station alignment boards" which the train attempts to capture directly beside a sensor. When approaching a station, the vehicle is slowed to allow the board to be captured by the onboard sensors, and the vehicle automatically brakes to a stop at a constant speed when it is seen. If the train passes the board, due to ice on the rails for instance, the train has to be manually reversed to capture it again. The stopping position of the train is especially important when the platform doors are used. Both sets of doors, train doors and platform doors must be precisely aligned, with just a few cm tolerance, to avoid the obstruction for passengers (specifically on wheelchairs). Platform doors are widely used on stations with expected large crowds of passengers such as in Shanghai Metro, Mekkah, Dubai and others.
In traditional train control, the railway is split into control "blocks" with signals in each one. Blocks are sized to allow the heaviest or fastest trains to stop fully within them. That way if a train is stopped in the next block, the following train will always have time to fully stop before reaching it. Careful tuning of block spacing is needed; if they are signals that are placed too close from each other, train speeds have to be reduced so they can still stop in time, but spacing them out further means the trains are also spread out and the route capacity drops.
SelTrac automatically maintains headway between vehicles through a moving block system. In this system the start and end points of the blocks are not fixed, and move along with the trains. This allows the central control system to calculate a point on the track where every train can safely move without further instruction—in a fixed block system this would be the next set of signals, but with SelTrac it is constantly being updated.
In theory this system could eliminate the "brick wall" criterion and allow the trains to run as close together as the communications speed would allow, but in practice a further spacing is typically imposed (for instance 50 m in the case of the Docklands Light Railway).
When originally being designed, computers were expensive and data storage was limited. In keeping with this model, the original SelTrac used on the ICTS centralized all control. After receiving location information from a vehicle, the blocks and safe target points for each were calculated, and this information was then broadcast back out through the inductive loop to be received by the vehicles. Onboard controllers used this information to calculate a safe speed to approach the next target point, and modified its current speed appropriately. The system was designed to reduce the complexity, and thus cost, of the vehicle controllers as much as possible.
In modern systems, much more information can be stored in the vehicle controllers. These now know the layout of the track, speed limits and various other data. This allows the controllers to make much better decisions about setting their speed—speeding up before an incline for instance.
SelTrac is offered in two ways:
1. A complete integrated solution in which movement authority and interlocking are integrated within wayside zone controllers; this reduces equipment and potential interfacing issues. Integrating the management of the interlocking with train location information, as communicated through the CBTC system, allows faster response times, more tightly controlled movements, and easier expandability and adaptability. Interfaces within the zone controller are more easily designed than those between subsystems. The integrated system knows the position of each train to a high degree of accuracy. It can control the behavior of the train at all times and, in response to changing conditions, can modify the behavior to ensure safety of the system while offering maximum service. It can adapt its algorithms to take advantage of individual train behavior, and change parameters to ensure optimum use of resources, such as platform availability and traction power. The limit of movement authority setting logic has a high impact on the end system performance, i.e. managing the interlocking in an integrated manner. Interlocking and switch control logic is optimized using position reports of communicating trains. The integrated design includes:
2. A 'progressive' overlay solution:
(a) Speed and signal safeguard: it vitally supervises speed profiles and signal adherence and provides all the functions of an intermittent ATP (automatic train protection)
(b) Continuous automatic train protection: provides the added value of improved headway while maintaining safe train separation, without depending on axle counters or track circuits. The system will automatically generate movement authorities based on actual locations of moving and fixed obstacles. It is used in conjunction with external interlockings and provides a replacement for automatic separation signals between interlockings. It can operate seamlessly with existing track circuits, which provides an inherent mechanism to operate mixed-mode traffic.
(c) Automatic train operation (ATO) offering automatic train movement allowing a driver to operate "hands-off" and thereby improve running performance.
Moving to fully automatic operation (FAO) provides an excellent means to match train supply to passenger demand.
Data communication is provided with either low frequency inductive loop or a high bandwidth, open-standards wireless system incorporating spread-spectrum radio technology.
SelTrac is installed in many railways around the world, including the following: