The proposed Purple Line, which is to connect Metro stations in Prince George's and Montgomery counties, will be the first state transit line built and operated by a private company, Gov. Martin O'Malley announced on Monday.

The state will seek a private firm to design, build and operate the 16-mile light rail line and its 21 new stations. The state would set the fares. All but $400 million of the project's $2.2 billion cost will be paid for by federal grants and private investment, according to O'Malley's office.

The project was part of $650 million in transportation projects for Montgomery County that the governor and Lt. Gov. Anthony Brown announced Monday in Bethesda.

The east-west Purple Line is among the highest-profile of the projects to be financed, in part, by the higher gas tax the Maryland General Assembly approved this spring. It is also the first project launched under a new bill written to encourage private investment in public projects.

Construction is expected to begin in 2015.

Motorists saw a 3.5-cent-per-gallon increase to the gas tax in July, the first increase of several scheduled over the next three years.