The line used to end at Sycamore, but now it will go three miles farther east to Mesa Drive.Officials say the three extra miles of light rail will add four stations and about 5,000 daily riders.
The new project should open on Aug. 22.
Valley Metro spokeswoman Susan Tierney says officials were able to work much faster than they anticipated in finishing the nearly $200 million project.
A test of the line will allow for a full inspection of the system, including track and overhead wire. It's also good training for workers.
Pedestrians and drivers in the area are being asked to be extra careful while workers test the track.
Roughly 50,000 people use the light rail each day.
The Associated Press contributed to this story.