The cities have drawn their lines in the sand. Dallas wants a second downtown rail line run underground, while a group of northern suburbs are demanding a 26-mile rail corridor that would connect them to DFW International Airport.

The problem: Each project costs more than $1 billion, and there isn't money for both.

On Tuesday, the board of Dallas Area Rapid Transit will decide which one gets priority.

DART's top brass has recommended doing both at the same time. But the long-range financial plan endorsed by staff leaves the subway dependent on $650 million in federal grant money that may not come through. The agency would take out debt to pay for the proposed Cotton Belt line through the northern suburbs of Addison, Carrollton, Richardson and Plano.

Dallas has made it clear that it doesn't want the Cotton Belt to be a DART priority. In a unanimous vote, the City Council told the agency to instead focus on the downtown subway, improvements to the bus service and a streetcar route in the central business district.

The city appoints seven of the 15 members in DART’s board and shares another with Cockrell Hill. The financial plan needs 10 votes to pass.

The board will meet at 6:30 p.m. Tuesday at DART headquarters.

Here’s what you need to know:

A second light rail path in downtown Dallas would break up a bottleneck.

All four light rail routes converge in the city’s core and share one set of tracks, limiting the number of trips. Last year, DART seized on a new federal grant opportunity and proposed a second rail alignment.

The plan was for the second downtown line — dubbed D2 — to run at street level from Victory Station, move underground as it entered downtown, rise to street level near Field and Young streets, and then head east along Young toward Deep Ellum. But the council nixed that route because of its impact on First Presbyterian Church of Dallas and a growing neighborhood of townhomes near Farmers Market. The council instead called for shifting the path from Young to Jackson Street.

This summer, DART staff said the Jackson route is not feasible.

Dallas leaders warn a street-level rail line will disturb downtown resurgence.

Business owners and urban planners are clamoring for a subway. They argue that any surface line will hurt economic development downtown, where the number of residents has quadrupled since 2004.

Council support for a subway is now unanimous. Burying the line, however, will double the cost of the project to $1.3 billion and delay its opening two years to 2023.

Subway advocates fear the Cotton Belt will endanger the downtown line.

DART wants the federal government to cover half the cost of D2. Supporters of the downtown project point out that DART's long-term debt will rise from $550 million in last year's financial plan to about $1.5 billion if it tackles the Cotton Belt now. They say that bleeding all that cash will hurt the agency's bond rating and its application for federal funds for D2, but DART officials said their financial metrics will remain healthy.

If the subway doesn’t get subsidies, it will be pushed back at least seven years, according to DART staff.

Until a few months ago, plans to launch the long-delayed Cotton Belt had been pushed to 2035.

Early this year, the rail line between DFW Airport and Plano was still two decades out even though DART had promised an east-west corridor since its creation in 1983.

Addison, a founding DART member, doesn't have a rail station and has been pressing for train service. This summer, DART proposed a cheaper version of the full Cotton Belt route that would lower costs from $2.9 billion to $1.1 billion. Addison, Plano and Richardson have committed to kick in millions of dollars extra so the line can open in 2022.

The proposed Cotton Belt (Staff Graphic)

Suburban leaders say the Cotton Belt will pump cash into the region.

The Cotton Belt will give the northern suburbs a transit connection not only to the airport but also to Fort Worth. A rail line between that city and the airport is scheduled to open in 2018.

Richardson Mayor Paul Voelker said those links will boost the region's economy by sparking denser development, like the $1.5 billion CityLine mixed-use district anchored by State Farm offices. The University of Texas at Dallas, also in Voelker's city, has planned apartments, shops and entertainment next to a future Cotton Belt station.

Addison has threatened to leave DART if Cotton Belt critics prevail.

Not everyone in the Cotton Belt’s path wants it. Far North Dallas homeowners have fought back. The Carrollton mayor prefers an enhanced bus line along the route — an option other suburbs deem inadequate.

If the people of Addison vote to leave DART, the town will lose bus service while remaining on the hook for its share of DART's debt. Mayor Todd Meier said the town council discussed options during a closed meeting Thursday but declined to elaborate about what they are.

People around the country are watching DART’s next move.

Transportation experts say there are no clear-cut answers when it comes to balancing the needs of the city core with those of the larger metro area. Adie Tomer, a fellow at the Brookings Institute think tank, said DART should be asking itself what its members want to get out of the system. Are they looking to make ridership spike through downtown or to get cars off the road at a more regional scale? Are they trying to encourage people to live denser?

Tomer called Dallas “a little bit of a cautionary tale” for the country, noting it was great at building rail everywhere but that ridership doesn’t reflect the investment.

Art Guzzetti, vice president of policy for the American Public Transportation Association, has a different view. He said Dallas is the envy of the country, even if its transit system "hasn't borne full fruit yet." Guzzetti cited DART's regional reach and its willingness to partner with app-based companies like Uber and Lyft to get people to their final destinations.

Staff writer Julie Fancher contributed to this report.