A string of fiber-optic sensors running along a 36-km stretch of high-speed commuter railroad lines connecting Hong Kong to mainland China has taken more than 10 million measurements over the past few years in a demonstration that the system can help safeguard commuter trains and freight cars against accidents. Attuned to the contact between trains and tracks, the sensors can detect potential problems like excessive vibrations, mechanical defects or speed and temperature anomalies. The system is wired to warn train operators immediately of such problems so that they can avoid derailments or other accidents.

At least 30 times during the seven-year period, the system detected anomalous vibrations. In a few cases, the vibrations turned out to be early warnings of dangerous emerging conditions that could have led to train wrecks. In some cases, vibration due to the use of the wrong type of lubrication oil in axle boxes was detected. The fiber-optic sensor system was designed for maintenance purposes and saves the rail company about $250,000 every year in maintenance costs.

The basis for the new sensor system is a technology developed in the 70s and 80s known as a Fiber Bragg grating, a type of sensor that reflects narrow spectra of light whose wavelengths shift due to temperature/strain variation. Coupling Fiber Bragg gratings with another device known as mechanical transducers allows pressure, acceleration and other parameters to be measured.