From TriMet.org

A vegetated trackway will provide a colorful carpet of low-growing plants along 200 feet of light rail line at the Lincoln St/SW 3rd Ave Station.
Eco-track at the Lincoln St/SW 3rd Ave Station Lincoln Station
In step with Portland's reputation as one of the nation's greenest cities, the Portland-Milwaukie Light Rail Transit Project is taking sustainable transit further with "eco-track" on SW Lincoln Street downtown. This vegetated trackway will provide a colorful carpet of low-growing plants along 200 feet of light rail line at the Lincoln St/SW 3rd Ave Station.
Although "green" or "grass" trackways exist in Europe, this stretch will be a Portland first by providing a vegetated trackway area pervious to stormwater, thus reducing runoff.
To sprout interesting colors and textures, SW Lincoln's eco-track will blend evergreen sedum types such as red carpet, green ice, orange ice and Murale, a sedum that changes from green in summer to red-orange in winter. Sedums are commonly used on eco-roofs thanks to their ease of maintenance, durability and shallow roots.
Custom grown in one-inch-thick mats, the sedums will be rolled out in rows between the rails and within the trackway. "They grow about three inches high so they won't interfere with the vehicles," the light rail project's West Segment Urban Design Lead Elizabeth Higgins explained. "Sedums are generally drought tolerant, but we will have a permanent drip irrigation system for plant establishment and in case they need water during the hottest summer months."
Higgins pointed out that the innovative design is part of a larger plan to enrich the urban experience along the light rail alignment. "We wanted to establish a new type of green street to contribute to a neighborhood known for its parks and greenery," she noted. "In addition to the eco-track, we are returning a canopy of trees on SW Lincoln as well as incorporating sidewalk stormwater planters."
http://www.trimet.org/pm/abouttheproject/index.htm#sustainability