Thermographic inspection

When heating a specific point or section of track, the distribution of this heat can tell a lot about the quality of the rail. Discontinuities in the track will influence the conductivity for heat flow. When following the heat distribution with an infrared camera, these discontinuities in the track can be found.  The main advantage of this system is, it can inspect larger lengths of rail at the same time when compared to the other methods. However, there is a tradeoff between the length of rail measured at a single given time and the amount of detail that can be gotten from the measurements.The disadvantage of this system is that a certain heating source is needed, which traditionally were often high powered lamps. Nowadays, the technique is also often combined with eddy-current systems, to induce local heating.

Inspection using visuals

Visual inspection can be done in two general ways. The ‘old fashioned’ way of simply sending someone in track (in person on foot or in a cart to visually inspect the track), or the more modern way of taking images of the track with a vehicle and analyzing this from behind a computer, as seen below (copyright TU Delft).

Both ways have their advantages and disadvantages. When going in track, you are really dependent on the weather. If there is a lot of sun, reflections might influence the possibility of finding small defects; on the other hand, raindrops can also mask defects. Another problem with in track inspections is safety. In many countries, visual inspections can only be done within a certain distance from the track, while the track is in normal operation. For more detailed inspections, one would have to go in at night, which gives additional problems with lighting.

Using images made from of a moving vehicle solves both of these problems. When watching the recordings on a computer, the visuals are much more constant and allow for close up inspection. However, the disadvantage is that the track can only be viewed from the predefined angles at which the images have been taken during the measurements. It is not possible have a complete view of the rail and its surroundings from all angles.

Laser measurements

Lasers can be used to scan the rail from a running train (usually a monitoring training). This can give information about the shape of the head of the rail, the exact distance between rails, the cant and many other parameters, depending on the system that is used. When combining this with other sensors, such as high-accuracy GPS receivers, it is possible to accurately determine the exact position of the rail and its horizontal profile. The most accurate systems are able to do this with an accuracy of about 1 centimeter. This makes using these combined measurements very useful for finding track subsidence, for example.

All of the monitoring techniques mentioned below are state-of-the-art. They are the newest innovations researched by universities and companies.

Instrumented crossings

Vibration-based condition monitoring is a technique which estimates the state of a component of interest by measuring its dynamic response. One of the benefits of this technique is that it is non-destructive. This technique can be used for cases where a known excitation is applied (for instance with a calibrated impact hammer), but it is also very applicable to situations where the input is not known. In this case, the technique is also non-intrusive, because the responses are obtained during normal operation.

This technique is therefore very suitable to monitor railway components, like crossings or insulated joints. These are impacted by every axle that runs over it, which creates high peak loads, but also results in a large dynamic response, which contains information about its state. Measuring these vibrations is done by attaching accelerometers to the components of interest.

When sufficient data is acquired, the measured response of each axle can be compared to a known response of the healthy condition and the state can be determined. By analyzing the higher frequency content of the signal, anomalies like surface defects can be detected. The content with lower frequencies typically contains information about the support condition of the rail components (like ballast voids) or structural defects (like loose bolts).

TUDelft patented ABA

The patented ABA system from TUDelft uses axle box acceleration measurements to find different problems in the track. With this measurement acceleration sensors are mounted on the axle boxes of a train. While driving at normal operating speeds, these sensors feel any excitation of the wheel due to disruptions in the rail. When analyzing the signals, both long range defects, such as temporal changes in the running band, and short range defects, such as cracks in the rail, can be found. These measurements have proven to give an accurate representation of the severity of defects. More developments currently are working on accurately predicting growth rate, thus allowing for better maintenance planning.

Photo of the ABA system sensors (Copyright TU Delft)

Video Gauge

With a video gauge measurement, it is possible to accurately track the movement of the rail at multiple locations simultaneously while a train is passing over. The system consists of a high speed camera with special software, which can track special targets. Using this system, it is possible to monitor transition zones or insulated joints, for example.

Currently this system is only being used for research purposes, due to pricing and the fact it needs an operator.

In the video below you see the video gauge measurement in action. A train is passing while the video gauge is scanning the targets and thus measuring displacement. The results are then displayed in matlab showing the vertical movement of the track during this passage.

Vibration measurements

Vibration measurements can be used to determine the spring constants (k-value) of the substructure. In order to do so, a vibration is generated, either on board of a measurement vehicle or on the rail directly. By measuring the response of the rail, the spring constants can be determined. These spring constants are essential for a good distribution of the load and keeping the track stable. An example of a vehicle capable of doing these measurements can be seen in the picture below.

Research tools

In order to perform research into the behaviour of the systems research tools are needed. Of course computer models are really important in this, however modelling is never a perfect representation of reality and can always have some errors (no matter how small). For this reason more research tools are available at the TUDelft.

Pantograph-Catenary measurement set-up

This test set up is specifically designed to simulate all the interaction between pantograph and catenary. Any type of pantograph can be installed in this set-up, after which detailed research can be done looking at the behaviour of the total system. Stagger motion, vertical displacement and even hard points in the catenary can be simulated in this set-up. In all these situations contact forces can be measured, but also the amount of contact-loss between pantograph and catenary can be monitored. Using this set-up better research can be done to optimize the system and thereby decrease the chance of failures. Also new types of pantographs can be tested in this set-up. The set-up is a hardware in the loop type of set-up, in which actual feedback from the hardware determines the behaviour of the response. This is necessary since the pantograph-catenary system has a high amount of interaction, making the behaviour of the catenary highly dependent on the behaviour of the pantograph.