Green track

Green track (also grassed track or lawn track) is a type of railway track in which the track bed and surrounding area are planted with grass turf or other vegetation as ground cover. It is a popular way of making railways more visually appealing, particularly for trams and light rail, and providing additional urban green space. Aside from the visual improvement, the vegetation provides a number of positive effects, such as noise reduction, less air pollution, rainwater runoff mitigation, and reduced urban heat island effect.
The first green tracks were installed in 1905 along Hardenbergstraße [de] in Berlin. Due to the technical challenges they posed, many green tracks were phased out in the following decades, but beginning in the mid-1980s, green tracks have seen a resurgence.
Green tracks have a number of positive impacts on the urban environment. Green tracks reduce surface runoff by retaining an estimated 50-70% of precipitation, while remaining stormwater is released more slowly and with fewer pollutants. The absorbed water is released through evapotranspiration and provides a cooling effect on the surrounding urban micro-climate. Vegetation, commonly grass or sedum, provides an important surface for the deposit and capture of fine particulate matter. High-vegetation tracks offer sound reduction under ideal conditions of up to 3 decibels, and are subjectively perceived as quieter.
A light rail project in Sydney, Australia identified green track as requiring 81% less concrete compared to embedded track.
The cooling effect of the green surfaces, particularly in summer, lowers the temperature of the rails themselves, reducing the risk of buckling. The city of Dresden estimates that their total installed capacity of green track has a cooling effect, reducing the temperature by 10 °C (18 °F) for 8.8 billion m (2.1 cu mi) of air per year.
Green tracks, particularly planted with endemic plant species, have positive impacts on the local biodiversity, such as the use of wildflowers that serve as food sources for wild bees, and providing habitats for insects.
There are several different designs for green tracks:
The ground cover requires a stable track bed construction, where the track geometry will not shift after construction. This is commonly accomplished with ballastless track. The addition of soil to track ballast would reduce the friction between the individual ballast stones and make it easier for the track to shift out of place, while making adjustments and corrections more difficult. At the same time, extensions and changes are more involved than with conventional track ballast.
The soil surrounding the tracks is planted with grass or other suitable ground cover plant. In comparison, grass tends to require higher maintenance, such as mowing and irrigation, and a soil layer of at least 15 centimetres (6 in). In particular, shade- and drought-tolerant species are chosen depending on local conditions; sedum is a common choice, as it requires a substrate of only 4–8 centimetres (1.6–3.1 in). The space between the tracks can also be filled with grass pavers instead of soil. The city of Braunschweig, Germany, together with the Julius Kühn-Institut, developed a mixture of endemic grasses and wildflowers suitable for green tracks that will also provide a habitat for wild bees.
Green tracks are particularly common in Central Europe; in 2015, Germany had a total of 565 km (351 mi) of green track. In France, the government prescribes the use of green track wherever practicable for tramway construction.