SIEMENS

Subways are the darlings of urban planners. They transport large numbers of people, take up little space, relieve congested roads, and reduce the impact of cities on the environment. But building them is expensive and takes years. Moreover, time and money are often in short supply in precisely those areas where modern transport solutions are most urgently needed – in the rapidly expanding megacities of Asia and South America. As a result, the construction of subways and light rail systems often becomes a bottleneck in transport systems.

But building new subway lines is no longer the only way to transport more people by train. A new type of train control system from Siemens based on a wireless local area network (WLAN) radio transmission channel makes it possible to use twice as many trains on existing lines and thereby transport more than twice as many passengers in the same period of time. “With conventional control systems, subways travel at intervals of approximately three minutes, but our technology reduces that to 80 seconds,” says Mattias Lampe of Siemens Corporate Technology China in Beijing, one of several locations where work is under way on the “moving block train control system with WLAN communication channel.”

The Chinese capital is a perfect example of the kind of city that can benefit from this innovative approach. Beijing’s subway system transports 7.6 million passengers every day – a number that has risen dramatically in recent years – and Siemens’ innovative train control system has made a key contribution to its success.

“To understand how we achieved this, you have to know how rail traffic is traditionally controlled,” says Lampe. The conventional method is to divide train lines into sections called “blocks.” Track sensors register when a train enters a section of the line and lock that section until the train has left it. “The principle is almost as old as the railroad itself, “says Lampe. This reliable system has made trains the safest mode of transportation there is. But there is a major drawback: Since trains have long braking distances and track sensors are expensive, the sections of the track that are kept clear for trains are also long, typically one kilometer in the case of subways. “It’s as though a traffic light waited until the road was completely clear up to the next light before turning green,” says Lampe. Cars also have a “safety block” that every driver maintains to the car ahead. “You could say that every car takes its own block with it,” says Lampe.

Can a train take its own safety margin with it too? It’s clear that train operators can’t drive by sight. The distances between trains are too large, and tunnels are too dark. So if trains are to have a “moving block,” the control system must be able to precisely determine the position of every train at every moment. The train and the control center must therefore be in constant contact, but it is difficult to guarantee stable wireless connections at high speeds or in tunnel systems, and such connections are also very expensive.

Now, however, Trainguard MT and WLAN radio technology are enabling a revolution in rail traffic. To implement the new control technology, Siemens engineers have installed radio transmitters along tracks and tunnels. Thanks to these “access points,” trains have a steady and reliable connection to the central control system and continuously transmit their position during each trip. “This makes it possible to calculate a train’s position on a track map to within a few centimeters,” says Lampe. The location data is compared with that of other trains so that required minimum spacing can be maintained at all times. “It’s like road traffic,” add Lampe. “If a train up ahead brakes, the one following it automatically brakes too, should such a response be necessary.”

In 2008, Beijing and Guangzhou became the first cities whose subway lines were equipped with the new system. Since then, demand for the technology has grown steadily. Trains are now using the “moving block” process in the megacities of Chongqing, Nanjing, and Suzhou. Preparations are also being made for installations in Qingdao and Xian. Subway systems in Istanbul, Copenhagen, Helsinki, London, and Hong Kong have also been upgraded.

Acid Test for Transmitters. “The concept has really developed into a mature product in China,” says Beijing-based Project Manager Xu Zhongliang from Siemens Rail Automation. It wasn’t easy getting there, though. “The crucial requirement is the reliability of the system,” says Xu. To eliminate risks, redundancies were built into it, he adds. The design of the WLAN system also ensures that signals from computers or cell phones carried by passengers can’t interfere with the system. The data is therefore sent not in a single stream but in a variety of data packets on different radio channels.

Field tests revealed some completely mundane challenges. For example, the WLAN transmitters have to withstand extreme environmental conditions. In particular, they have to cope with dirt, rain, heat and cold. In addition, it was not possible to fall back on prior experience when it came to establishing the right distances between the individual access points. These were ascertained in cooperation with CT China researchers. Access points are currently mounted at intervals of approximately 250 meters and connected to one another and to the control system via fiber optic cables.

“At this point, the systems are operating with a high degree of reliability,” says Xu. The conventional control system with fixed blocks and sensors installed in the track bed is still used as a back-up system and to operate maintenance trains.

The new system is not only an ideal solution for refurbishment of existing metro lines but also a cost-effective way to quickly increase capacity, ease road congestion, and minimize adverse environmental impacts. In theory, it could be used to run trains fully automatically, even though most subway operators still rely on train drivers. However, the drivers usually just supervise the train operation. They only have to intervene if an emergency arises.