Maryland transit officials have scaled back design work on a new transit line to connect upper Montgomery County with the Metrorail system, saying they now are focusing on a nine-mile busway rather than the original 15-mile route.

The nine-mile segment would connect the Shady Grove Metro station with the Metropolitan Grove MARC commuter rail station. Buses running in their own lanes would serve King Farm, the Kentlands and emerging developments in Rockville and Gaithersburg west of Interstate 270.

Meanwhile, design work on a six-mile section planned to extend farther north to Germantown and Clarksburg has been postponed until those areas grow enough to make a transit line cost-effective, state officials said. The Maryland Transit Administration’s Web site now refers to the northern segment as a “six-mile future extension.”

“We’re not abandoning the northern portion,” said Rick Kiegel, the state’s manager for the transit plan. “We’re just advancing the project in phases to give that northern portion a chance to develop.”

Kiegel said there is no funding or schedule for work on the Germantown-Clarksburg segment to resume. However, he said, the entire 15-mile route will remain in long-term growth plans to ensure the right of way is preserved.

Map of most recent Corridor Cities Transitway plan

If construction funding is secured, the state plans to break ground on the first segment in 2018 and open it in 2020, Kiegel said. The southern section is estimated to cost $545 million in 2012 dollars. The northern part is projected to cost $300 million.

State transit officials have said for years that the transit line, known as the Corridor Cities Transitway (CCT), might need to be built in segments. Such an approach is common for large, expensive projects, including the Metrorail Silver Line, which is being built in two phases in Northern Virginia.

However, the Maryland transit agency didn’t announce that it had decided to do so on the Corridor Cities Transitway. Kiegel said the state changed its Web site’s description of the project in February to a nine-mile line.

Marilyn Balcombe, president of the Gaithersburg Germantown Chamber of Commerce and a leading advocate for the transitway, said she heard of the decision only recently, after state officials began releasing details of how they would allocate revenue from a new state gas tax.

Maryland Gov. Martin O’Malley (D) announced $100 million to fund more design work on the Corridor Cities Transitway when he signed the gas-tax legislation in May.

Balcombe said the business community welcomes the state’s commitment to expand public transit to west Gaithersburg, where county growth plans call for more density in Montgomery’s biotechnology employment corridor.

However, she said, she’s concerned about a chicken-and-egg effect: While the state waits for Germantown and Clarksburg to grow before it builds the northern segment, companies might hesitate to build in or move to the area as long as a transit link remains uncertain.

“Having the CCT funded and on the books would be a great draw for density and future development,” Balcombe said.

Kiegel said limiting work to the first segment will enable the state to pursue the project in addition to two other state transit priorities: building a 16-mile light-rail Purple Line between Bethesda and New Carrollton and a 14-mile light-rail Red Line in Baltimore. Those projects are estimated to cost more than $2 billion each.

The state is seeking highly competitive federal construction aid for all three proposals but would be required to pay for at least half of each.

Companies with development plans in Rockville and Gaithersburg, including Johns Hopkins University and Shady Grove Adventist Hospital, have advocated strongly for the southern segment because county growth plans limit much of the proposed building until a transit line is funded.

Some developers in Germantown and Clarksburg said they were disappointed, but not surprised, to hear about the state’s focus on a southern segment. They said they realize the state has limited transportation funds and that Rockville and west Gaithersburg have far more jobs and office parks.

However, Campbell Smith, of the Trammell Crow Company, which is planning a 54-acre development in Germantown, said stopping a transit line at Gaithersburg would leave too many commuters stuck on already jammed I-270.

“If part of your goal is to build public transportation to alleviate traffic . . . and a lot of employees live to the north, why wouldn’t you extend it?” Smith said.

Michael Smith, of the LCOR development firm, said his company will proceed with plans to build apartments, office space and retail near Clarksburg, regardless of when the transitway reaches them. The firm owns 203 acres near I-270 that includes the Comsat building, where the northern segment would end.

Michael Smith said he hopes the success of a southern segment would improve the chances of the line being completed.

“It’s good to start where the population base exists and there’s the density to sustain the argument that this is a good thing to invest in, that it makes sense and it works,” he said.

County spokeswoman Esther Bowring said the county supported building a transitway in two parts to get construction moving before waiting for the entire line to be funded.

“We’re operating under the assumption that phase two will happen,” she said.