Metro is finally, and literally, on track to finish the Green Line light rail project.

The first train passed over the Harrisburg overpass as part of a "cold slide" test meant to make sure the train is not obstructed and platforms are flush with the doors and floor of the trains all the way to the Magnolia Park Transit Center. During the test, the train is pulled along the tracks and protected with Styrofoam in spots. Essentially, officials just certify they didn't hit anything.

READ MORE: Harrisburg overpass taking shape, finally

The overpass is the final piece of the $587 million Green Line, which was completely paid for by local taxpayers as part of the 2003 Metropolitan Transit Authority referendum. A roughly two-mile segment along Harrisburg from downtown Houston to Altic opened in May 2015, at the same time as the $823 million Purple Line along Scott and Martin Luther King.

Work on the Green Line was slowed by progress on a freight rail overpass along Harrisburg near Hughes. After the community and city officials urged Metro to build an underpass, transit officials agreed, only to renege when environmental concerns made an underpass unlikely.

READ MORE: Metro waiting for heat to die down on rail delays

After sometimes contentious design meetings with the community, already frustrated by years of light rail construction that cut off businesses from customers, construction of the overpass lagged. Metro, with some officials calling it "the project from hell," begrudgingly agreed to a revised schedule and $600,000 in incentives to the overpass' builder, McCarthy Building Companies, to finally get the span completed and reopen lanes on Harrisburg.

The start of testing, while a milestone, does not mean service along the entire route is imminent. Months of testing of both the tracks and electrical system are needed before passengers can ride to Magnolia Park. The state of revenue service is expected in late January, possibly just days before visitors come to Houston for Super Bowl LI.