San Francisco's iconic cable cars resume service after upgrades

San Francisco's iconic cable cars are back up and running after two weeks of upgrades

Muni has been working on gearboxes inside the cable car barn that powers the entire system. 

The gearboxes spin 30-foot wheels that pull the 12 miles of underground steel cables that propel the cable cars up the city's famous hills. 

Muni provided bus shuttles to replace the cable cars while upgrades were being made. 

Managers say the work was needed to ensure the safety and reliability of the system.