There is public interest in the Porter Square red line rail station getting a link to green line rail, a state transportation official says. (Photo: Marc Levy)

A link between green line and red line trains in Porter Square could be in the future, Katherine Fichter, project manager for the Green Line Extension project, said at a Wednesday project update in Cambridge.

The far, far future, she said.

“The extension that we’re building that’s currently going to terminate in Union Square, there could be a future in which that was then extended to make the connection for the red line and commuter rail at Porter. That’s not part of this project,” Fichter said before the meeting at the Kennedy Longfellow School. “But there’s nothing we’re doing to make it impossible … we are not doing anything to preclude that.”

There was public interest in exploring the link, she said.

Plans to extend the blue line 1,500 feet to meet the red line at the Charles/MGH stop in Boston are under way, with environmental reviews and final designs to be complete by Dec. 31, 2011.

The Union Square spur off the green line stop at Lechmere in Cambridge has a Dec. 31, 2014, deadline, along with several other new stops through Somerville into Medford.

But the idea of linking the red and green lines at Porter is new — perhaps because there hasn’t been a Union Square stop from which to link.

The Massachusetts Bay Transit Authority asked for transportation suggestions from the public in 2003, a “financially unconstrained” exercise that drew about 400 proposals and launched the green line extension.

A proposal to connect the green line to the red line, but at Harvard, didn’t survive, authority spokeswoman Lydia M. Rivera said.

“This idea was raised … but was screened out,” Rivera said in an e-mail in 2005. “The MBTA felt that there was not sufficient demand for this service, and other proposed projects in the area would meet any potential demand.”

Not precluding the “urban ring”

The idea of an “urban ring” connecting MBTA stations far outside Boston, instead of having riders come into the city to switch lines, has existed at least since the 1970s, but the state acknowledged in January that it doesn’t have the money to pursue the plan.

Fichter’s language in answering whether planning for the green line extension would enable connection with the urban ring, whenever it came about, was similar to her answer on a red line link: “Again, we’re working hard not to do anything to preclude it,” she said.

Whenever it’s tackled, plans are for the urban ring to be served by buses, not rail.

In explaining why the green line extension is rail instead of buses, Fichter said that switching would mean “saddling your riders with a change at Lechmere, which is something you always want to avoid. The optimal solution is to continue the existing mode.”

“We have existing green line travel to Lechmere and rail rights of way,” she said. “East Cambridge, Somerville and to a somewhat lesser extent Medford are already well served by MBTA buses. They don’t operate particularly well in the environment given the corridors, the congestion, the density. So that’s when you begin to think that a different mode would be more appropriate, and in this case we have the rail opportunity.”

“It’s not like we have a mode-specific policy,” she said.