Automatic Train Control System (ATCS) Technology
Muni Metro is the third busiest light rail system in the U.S. But it’s the last to funnel five lines into one tunnel, making automatic train control vital to manage our trains effectively.
Our current Automatic Train Control System (ATCS) only exists in the subway, which means Muni Metro trains operate under the control of the train operator on San Francisco streets.
The ATCS operates trains in the subway, keeping them safely and appropriately spaced apart by controlling train speeds, switches and signals. The operator is always prepared to take over if necessary. Meanwhile, staff at the Transit Management Center oversee all train movement and activity, providing bird’s-eye-view guidance to operators as needed.
The ATCS is composed of central computers, and computers on each train and along the side of the street, the software that tells the system what to do and the communications network that allows the computers to communicate with each other.
At night while no trains are running, staff reboot the ATCS computers and load the software from a set of floppy disks. Early in the morning, an out-of-service “sweep train” moves through the subway to ensure the trackway is clear and there are no disruptions to the ATCS components that communicate with the trains.
Trains have computers on board that communicate over a low bandwidth connection to an antenna mounted between the rails. This connection is slower than a dial-up modem. The trains connect to the system each time they arrive at subway portal entrances, like logging into your computer each time you do work.
Operators must guide trains into the subway at a very specific speed for the ATCS and the train to successfully communicate. Sometimes the train needs to stop and wait for confirmation from the system that communication has been established. Occasionally this process fails, the train and the ATCS don’t communicate and the train must travel in manual mode at half its maximum speed, which also slows all other trains along the network.
When trains exit the subway, the operator switches back into manual mode to continue its trip on the street where there is no ATCS technology.
History: Moving Block Technology
Dating back nearly to the invention of railroads, railways were divided into segments called “blocks.” Automatic train control is also called a “moving block system” where a “block” is the space the train occupies. With automatic train control, the “block” encircles a train and moves with it, like a bubble. As one train’s moving block approaches another train’s moving block, the approaching train automatically slows down and eventually stops so the blocks do not overlap. Moving blocks also change size based on the speed of the train to account for the extra space it takes for a train moving at higher speeds to stop. A slow train will have a smaller block, and a fast train will have a larger block. Moving block systems allow train control software to safely pack trains more closely together and enable greater frequencies and capacities than older fixed block systems.
Modern Communications-Based Train Control (CBTC) Technology
A modern communications-based train control (CBTC) system will improve on the benefits of automatic train control in the subway and extend the technology to on-street service from end to end. It will enhance Metro safety, reliability and efficiency citywide with fewer system failures. The new technology will track all trains from the start to end of service, anticipate streetlights earlier to give trains the right of way and reduce delays, and make track switches easier, among other service and operations benefits.
Communications-based train control (CBTC) technology is still a relatively recent development in transit technology. At the time it was purchased and installed, SFMTA’s ATCS was one of the first computerized train control systems in the United States with a useful life of 20 to 25 years. This infrastructure has been meticulously maintained but not meaningfully upgraded over the past three decades in part due to inadequate funds for Metro maintenance and state of good repair.
Replacing the existing train control system is imperative now that multiple components are becoming obsolete, and reliability is beginning to suffer due to the system’s age. Today’s technology is now leaps and bounds more advanced and powerful, and while we expect our investment in a modern system to last another at least another 20-25 years, we also have a lifecycle support plan that ensures components will keep pace with future advancements in train control technology.