The Arizona Department of Transportation said last week it will complete a route study for a commuter rail line linking Phoenix and Tucson by the end of 2013, but no funding source has yet to be  determined that could provide the $1 billion that such a project is expected to cost.

AzDOT is looking at six potential rail routes linking downtown Phoenix, Phoenix Sky Harbor International Airport, and Tucson International Airport as well as a system of express buses.

The passenger rail lines would be up to 128 miles long with as many as 11 stops.

The $6.3 million study, which began in 2011, is looking at using tracks operated by Union Pacific Corp. and BNSF Railway.

Most of the proposed routes generally follow Interstate 10 between the cities.

Potential funding sources include an extension of the Proposition 400 sales tax in Maricopa County and revenue generated from Proposition 204 on the November ballot, which would provide up to $100 million a year for statewide transportation efforts.

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