In the Stockholm metro, microphones on the underside of trains will listen for irregularities in the rails. The aim is to maintain the tracks more efficiently and make the trains run quietly.

Written by: Linda Nohrstedt  - Nyteknik

Seven metro trains in Stockholm are being equipped with the Quiet Track system. Here you can see the pulse sensor and accelerometer mounted on the wheel axle. 

The system is called Quiet Track and has been developed by engineering consultancy Tyréns in collaboration with several other actors over three years. It has been tested on one train on the green line in the Stockholm metro, but now the county council's traffic administration has bought the service for the entire metro system.

Seven trains will be equipped with microphones, pulse sensors, accelerometers and GPS. Each train will also be equipped with a computer where the initial signal processing takes place before the information is sent to a central database.

From September, the seven trains will start running with the new monitoring system. The microphones will continuously pick up the sound of the trains running on the rails.

But to determine the condition of the track, it is important to distinguish the sounds coming from the rails from those coming from the wheel. This is why an encoder is used, which emits thousands of electrical pulses per wheel revolution.

- The sound that keeps coming back is the wheel contribution, while the track sound is something new. In this way, the wheel contribution can be excluded from the measurement. This is technology that exists in many other industries, but we haven't seen anyone else apply it like this and make it work," says Martin Höjer, civil engineer and acoustician at Tyréns.

The idea of Quiet Track is to map wear and noise so that measures such as grinding or lubrication can be implemented quickly.

The traffic administration in Stockholm County Council has high hopes that maintenance will now be more efficient. For example, micrometer-high irregularities in the rails, which can cause noise, can be easily detected.

But other maintenance can also be facilitated by the system. For example, Quiet Track continuously assesses the stiffness of the track to determine whether it is time to replace the so-called rail pads, rubber mats under the rails. If the track is slippery or has been subjected to dangerously heavy wear, the system will also sound an alarm to avoid accidents.

Tyréns also sees the possibility of combining Quiet Track with automatic lubrication systems mounted on the train so that the track is lubricated as the vehicle passes.

- "Our system could easily tell such a lubrication system where and when to spray lubricant. Then the rails could be lubricated the second the wear is detected," says Martin Höjer.

Formally, the system is called Quiet Track Monitoring System, QTMS. It has been built in collaboration with IBM, whose internet of things platform is used for cloud-based data storage, alarm generation and graphical presentation of measurement data.

Each train is equipped with four microphones, two accelerometers and an encoder. The accelerometers are located on the wheel axle and measure the upward and downward movement of the train to determine whether the vehicle is passing a junction or a bump. The encoder is located on the axle and emits 4 000 electrical pulses per wheel revolution.

Positioning is done with GPS and supplemented with RFID technology and sensors in tunnels.

Stockholm County Council has purchased QTMS for three years, at a cost of just over SEK 6 million, with the option to extend twice for two years each.