Photonics Keeps Trains on Track

Brent D. Johnson

Traffic controllers for the national railway system have a problem. They have to move some 550,000 freight and passenger cars along 173,000 miles of track, frequently without knowing the precise location of a train or which way it is headed.

The US Department of Transportation in conjunction with the Coast Guard, the Federal Railroad Association and others is implementing a differential global positioning system network to monitor railroads throughout the US. This system can locate trains to within 10 feet, but it does not indicate direction of travel. Nor does it provide continuous positioning information, because signals can be occluded by tunnels, bridges, mountains, buildings or foliage. In addition, with its margin of error, the system is inadequate for discriminating between parallel tracks that can be as close as 11.5 feet.

As part of its positive train control program, Lockheed Martin has developed a location determination system that provides precise real-time position, speed and heading data. Using a patented algorithm, it integrates data from the global positioning system and from accelerometers, tachometers, a track database and KVH Industries' E*Core 2000 fiber optic gyroscope, which tells exactly where the vehicle is headed and how its motion is changing from second to second.

James Archibald, manager of engineering for Lockheed's Niagara Operations, said that the company selected fiber optic gyros from KVH because they offer good performance in an off-the-shelf package.

The primary benefit of these gyroscopes, he said, is that they have no moving parts. Traditional gyros use a bearing or disc spinning in a low-friction liquid, while fiber optic gyros launch light through a fiber optic coil. As the light enters the gyro, it is split into two beams that trace opposing paths around the internal circumference of the device. When the device is motionless, the light is in phase when the two paths reconnect. If it moves, the light doesn't recombine properly, and the interference pattern that results can be measured. The E*Core 2000 can measure angular rates up to 100° per second, with a resolution of 0.01° per second in a 100-Hz bandwidth.

The information from the location determination system is used to predict safe braking distance in real time and to assure that the engineer does not exceed his limits of authority, in terms of speed, stopping points and itinerary. The system is being used in a train control project by the Illinois Department of Transportation, but the Federal Railroad Association is pushing for nationwide implementation.