State transportation officials have unveiled a lower $2.3 billion cost estimate for the proposed extension of the Green Line to Somerville and Medford, but the future of the long-delayed project remains in doubt.

The state transportation board and the control board that oversees the Massachusetts Bay Transportation Authority were meeting Monday to discuss the viability of the modified plan.

A proposed redesign would include simplified stations and bridges.

State Secretary of Transportation Stephanie Pollack says even with the revised cost estimate, a number of questions must still be answered before construction can begin.

The Green Line Extension, which was originally tied to Boston's massive Big Dig highway project, was put on hold last year after new cost estimates put it up to $1 billion above the original $2 billion estimate.