Starting today, the Mesa light rail extension will start running — seven months ahead of schedule. Construction on that line began January 2012.

On the other side of town, crews continue to work on the northwest extension — the line covering a stretch of 19th avenue. That line is not expected to be completed until 2016 — on time, but not ahead of schedule.

Both lines are just over 3 miles long. Corinne Holliday with Valley Metro said the northwest extension posed some additional construction challenges.

"Very early on, the northwest Phoenix light rail extension there was a lot more electrical lines that needed to be relocated so that took a little bit of extra time than anticipated, " said Holliday.

Holliday said around 500 miles of underground utilities had to be moved to accommodate the line, which runs from Bethany Home Road to Dunlap Avenue.

Updated 8/21/2015 at 10:45 a.m.