Public transport by rail or tram has over the last decade become a popular way of transportation. The number of passengers taking advantage of a comfortable and safe ride is constantly rising which calls for higher train speeds and shorter stop intervals, exposing the rail- and tramways to extreme mechanical stress.
Unavoidably wearing out in time, not only annoying, but also dangerous, defects start tweaking in. A new and systematic maintenance solution (Fig. 1)  supports the monitoring and maintaining for rail- and tramways.
Fig. 1: A systematic rail maintenance concept includes measuring, locating, planning and fixing rail defects.
Analog Devices Blackfin processors and National Instruments LabVIEW plays a central in the system, securing all measurement and field-data correctly to be stored for immediate action. The result is clear, much longer operation times between exchanging rails, ensuring the public transport service a more economic and successful evolution than ever before.
Railway Tracks "Under the Hood"
When new rail- and tramways are laid out, high quality assurance verifies correct track positions prior to concreting. Some time after installation, several defects start to tweak in to the rail parameters (Fig. 2) during the daily operation.
Fig. 2: Rail parameters are divided into track geometry, longitudinal profiles and cross-sections.
This is due to the mechanical contact between the wheels and the rails in relation with a highly complex dynamic spring-mass model ranging from the train chassis to the railway underground. The defects, their critical parameters and tolerance windows are classified by railway engineering standards .... It's the goal of this rail maintenance program to deal with the irregularities but keep them to acceptable levels.
Rail Track Geometry
The track gauge is the distance between two rails and responsible for the so-called "sinusoidal ride" of the train. This keeps the spot where the wheel and rail meet constantly moving to minimize wear-out.
Variances in the track inclination can make passing trains shake and shudder. Mostly caused by giving way of the railway underground, inclination defects can also be kicked off by surface irregularities such as corrugations and holes. Systematic inclination profiles are however necessary to minimize accelerating forces to the passengers when a train is riding in and out of a curve. A correct track-to-track distance prevents any chance of collision when trains are crossing at high speed.
Longitudinal Surface Profiles
Cracks and breakouts are among the most feared since they can lead to catastrophes such as derailing. Corrugations on the other hand are wavy irregularities with a characteristic wavelength between 20 to 100 mm and are annoyingly noisy when their amplitudes exceed 0.05 mm. From 0.3 mm peaks however the vibration can leave irreversible damage to the railway bed.
It's also in their nature to move along the rails and scientists still debate where they originate. Single holes are mostly generated by turning or jumping wheels and follow the mathematical equation of a polynomial. They're responsible for the sudden bumps on a tramway ride. Regular bumps that are often experienced on older railways are due to the welding interfaces of the 18-m railway sections.
The head geometry of a newly installed rail follows an exactly calculated contact geometry which optimizes the critical wheel-to-rail interface. The shape consists of tangential lines and specific radii allowing the wheel to economically and smoothly roll off with a safe horizontal guide.