Gov. Martin O'Malley has long promised that Baltimore and the Washington suburbs would each get a new light-rail line and that the Red Line and its Purple counterpart outside D.C. could be built at the same time.



But state financial documents recently submitted to the Federal Transit Administration show that O'Malley's promise, to the state's most populous regions, will be difficult — if not impossible — to keep.



The General Assembly's recent rejection of the governor's proposed gas tax hike makes it increasingly likely that the state will have to choose to build one line before the other, state and local transportation officials say. With no new tax revenue dedicated to transportation, finding the money for even one of the light-rail lines will be difficult, the officials say.



The state hoped to begin construction on both lines in 2015, with the 14-mile Red Line — which would ultimately run from Woodlawn to the Johns Hopkins Bayview Medical Center — opening in early 2021 and the 16-mile Purple Line opening between Bethesda and New Carrollton by late 2020. But construction of the runner-up project probably would be delayed at least five years, until after the chosen line is built.



When asked whether the state could afford to build both at once without a gas tax increase, state Transportation Secretary Beverley Swaim-Staley said, "The short answer is probably no. ... Obviously, it would have a significant impact on our ability to build anything else."



Time to find additional money is short. State and local officials say they have 12 to 18 months to prove to the Federal Transit Administration that Maryland can pay half the cost of constructing one or both lines. Otherwise, the state could fall behind in the rigorous competition for federal transit money.



Officials estimate that it will cost $1.93 billion to build the Purple Line and $2.2 billion to build the Red Line.



Post staff writer Aaron C. Davis contributed to this report.