(Photo courtesy of Phoenix Sky Harbor International Airport)

Phoenix Sky Harbor Sky Train.

The monorail system at the Phoenix Sky Harbor Airport is seeing more daily passengers. Since 2013, ridership has increased by about 30 percent.

Julie Rodriguez, an airport spokesperson, said the increase is due in part to the extension of service from Terminal 4 to all airport terminals. She said it's stage two in a multi-step construction plan for the Sky Train system.

"One reason that the riders are up on the Phoenix Sky Train is that the buses between the terminals have been eliminated, so we have passengers taking the sky train between Terminals 3 and 4," Rodriguez said.

Right now, about 13,000 people ride the Sky Train each day. Officials said the next stage will extend service to the rental car center.