Proactive maintenance and other mitigation measures can reduce the noise emitted into surrounding areas from the passing trains on the SkyTrain guideway, according to a new report released this week by TransLink.
The public transit authority released the full findings of its SkyTrain Noise Study, independently coordinated by SLR Consulting, which conducted measurements of noise at 30 locations along the Expo Line and Millennium Line in Spring 2018.
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With numerous factors accounted for, including residential density, the greatest problem areas for noise have been identified as the spans between VCC-Clark/Commercial-Broadway and Nanaimo stations, which has the highest number of people potentially affected by “very high noise levels” both above 85dBA and above 90 dBA. This largely has to do with sections with rough rails in high-speed sections of track, especially as trains cross switches.
Other top problem areas are located around Joyce-Collingwood Station, the span between Edmonds and Columbia stations, the span between Patterson and Metrotown stations, and the span between Gilmore and Brentwood stations.
According to the technical consultants, four main factors combined result in the excess noise issues:
- Train speed over high-speed sections of track is a leading factor, with the maximum normal operating speed being 80 km/hr and a catch-up speed of 90 km/hr.
- Building proximity, especially residences, to the tracks.
- Elevation of buildings, with the highest noise levels at locations overlooking the guideway, particularly residential towers.
- Track condition, as rail roughness, corrugation, track defects, or worn switches result in noise levels that can be upwards of 15 dBA greater than other locations with track and switches in good condition. Minor breaks or tiny gaps cause vibrations in the wheels, rails, and other track components, which are heard as noise. Even smooth wheels and rails produce sound due to microscopic roughness.
Weather conditions, including precipitation and temperature, potentially have a minor impact to track noise.
As there are also three different generations of trains on the Expo and Millennium lines, differences in wheel design potentially produce differing noise impacts, especially when trains pass over switches.
Longer trains are potentially also a factor for noise. For instance, the 24 axles on the original Mark I trains, now typically operating in a six-car train formation, will result in 50% more impacts through a worn switch than the Mark II or new Mark III trains, which run in a four-car formation (using longer cars) and only 16 axles per trainset.
The study did not look into the noise experienced by passengers inside trains, which can differ between train models, with varying designs in wheels, windows, doors, and insulation.
Analysts also indicate they have established a train passby noise goal of 75 dBA at residential facades with windows closed, which is “considered to be a reasonable balance between the adverse effects of noise and other benefits of rail transit systems to communities.” Depending on glazing and insulation, according to the World Health Organization, noise levels typically reduce by 30 dBA with windows closed.
To address excess noise issues, TransLink states an improved maintenance of switches is necessary.
“These switches wear over time and cause increased noise levels as the transitions from the switch to the rail become less aligned with one another,” reads the report.
When rail needs to be replaced, harder rail preventing noise-emitting corrugation could be used instead of the softer rail steel installed on the original sections of the SkyTrain. This is already in place on sections of rail that were recently replaced.
TransLink is consulting with other transit agencies that use harder rail steels, but preliminary findings indicate the benefits of reduced track noise may be negated by a quicker rate of wear on the train wheels.
Other practices to mitigate noise include “friction modifiers” applied onto the tracks as a liquid or in the form of “solid sticks” pushed against the wheel treads, which can reduce rail wear and slow down corrugation, as well as improved acoustic rail grinding during maintenance time.
Rail dampers could also be considered to absorb vibrations, but this is deemed to be expensive to install. Continued tests with dampers have been completed and are likely to continue into 2020.
The study and the latest tests on noise-mitigation options, examining the results, will eventually lead to the implementation of a new noise-mitigation plan.
Additionally, TransLink is in the process of finalizing a contract for a consultant to lead the development of better acoustical insulation guidelines for new buildings constructed near the tracks. This work will be performed with municipal governments.