Commuters who hope that a 10-year transportation sales tax on the ballot next year will bring them train service far into the suburbs in the next decade may be out of luck.

Some of the highest-profile projects proposed for a transportation sales tax may not be capable of completion by the time the tax ends, weakening their chances for inclusion in the referendum. While the projects are not disqualified, if they cannot be finished before the tax ends they will be less attractive to voters.

Local advocates for major transit lines, which they hope could be rail, into Cobb County, across the top end of I-285 and into Gwinnett County said state analysts in preliminary discussions had told them those lines likely could not be complete and running by the end of the tax’s 10-year period. The project advocates emphasized that the state had not finalized its report and did not draw absolute conclusions. The report is scheduled to be out Thursday.

In 2012, the 10-county Atlanta region will vote on a list of projects in a referendum, and on a 1 percent sales tax to fund them, to run 10 years. A group of local elected officials called a roundtable is about to start choosing projects for the list. It is up to them what to choose from the $22.9 billion pool of eligible projects, but they must consider whether a project is likely to be finished on time.

Norcross Mayor Bucky Johnson, who chairs that group, acknowledged the issues facing some of the transit projects. "It would be good if you could just add water," he said ruefully.

Speaking only for himself, Johnson said he could imagine placing "some" unfinished projects on the list -- for example, to complete land-buying for a future rail line while commuter bus service fills the gap -- but not many. "The fact is that when people pay the money, they want to see something for it," Johnson said. "Especially the first time, they want to have something they can touch."

The problems facing the transit projects include geographic ones like having to cross a river -- something the transit line into Cobb County and the line across northern I-285 will have to contend with -- staff for local governments and tax districts said.

A proposed Gwinnett County line runs into issues because its first leg shares land with a railway company, Norfolk-Southern.

“I think you could get the whole thing done in 10 years, personally,” said Chuck Warbington, executive director of the Gwinnett Village Community Improvement District, which has advocated for the line. “But the folks reviewing this have a lot of experience.”

In the absence of a sure thing, some of those advocates suggested a small first phase might be done instead.

Yvonne Williams, president of the Perimeter Community Improvement Districts, which is advocating for transit along I-285, said that if there is not time to implement such a massive new development as light rail infrastructure along I-285 from Doraville to Cumberland, buses could jump-start the project. That would pave the way for ridership on a more substantial project later. “We should phase it,” she said. “Bringing it online early would make the voters see the benefits.”

The project advocates stressed that equally important data should not be overshadowed: the high numbers of people their lines could serve.

Faye DiMassimo, director of Cobb County's Department of Transportation, noted that the northwest corridor transit line scored in the top tier of a study done on transit impact at the Atlanta Regional Commission. The state has said that that line is likely to be in the middle of construction at the time the tax ends, said Dan McDuff, her deputy director.

Indeed, such a project's impact can dwarf that of a smaller one-stop line, or streetcar project that could be easily built in a few years. The roundtable will have 66 transit projects to pick from, totaling $14 billion. The vast majority of those will not make the final list voters see, since the tax will only bring in about $8 billion. The list also must include roads and bicycle or pedestrian projects. There are 326 roadway projects worth $8.6 billion to choose from.

Todd Long, the state's transportation planning director, said that few if any of the road projects on the list would face the same issues as brand-new long transit lines. Even the $500 million proposal to rebuild the interchange at Ga. 400 and I-285 is likely to be finished within the tax's limits, if started early, he said.

Part of the problem is that Georgia has few transit projects in the pipeline towards construction -- a long journey that includes expensive, time-intensive tasks like design, federally mandated studies and land acquisition. The state gas tax legally can only pay for roads and bridges, not mass transit, so there has not been much funding for such activities.

The Georgia Regional Transportation Authority, which is performing the risk analysis for finishing the transit projects on time, declined to confirm the results prior to the public release Thursday.  However, Jannine Miller, director of the authority, stressed that it had been done with deep research into data, all with the view of making sure that citizens felt at the end of the 10-year tax that government had kept its promise.

And should the roundtable and the voters choose some of those long-haul projects anyway, they would have good reason, she said. "They know that this sales tax revenue will be the only robust revenue stream for developing transit that we’ll see in the next decade," she said. "This is our chance to take a bite of the apple and advance a transit buildout that wouldn’t otherwise be possible."