BOSTON (AP) – Passengers on Massachusetts Bay Transportation Authority subway trains will soon be able to use their cell phones and personal digital assistants while underground at four downtown Boston stations.

The agency’s board of directors has reached a 15-year contract with InSite Wireless of Alexandria, Va. to provide wireless service at the Park Street, Downtown Crossing, Government Center and State Street stations as well as the tunnels that connect them.

The stations are all within a half-mile radius and are among the busiest in the system.

The service, which is expected to be up and running by the fall and will eventually expand to other stations, is not just for convenience, but for safety as well.

“This will be a new convenience for our customers,” MBTA general manager Michael Mulhern said on Thursday. “And it will be a benefit to safety and security, so we’re excited about it.”

InSite, which beat out four other companies for the contract, thinks all the major voice and data providers will sign contracts to use its system of underground antennas and fiber-optic cables to reach their subscribers.

T-Mobile already provides service to its customers on the platforms of the four stations.

T officials said installing wireless service in subways, a recommendation of the agency’s anti-terrorism task force, will enhance security by allowing passengers to call for help without having to rely on police call boxes in the stations.

Daniel A. Grabauskas, the state secretary of transportation and chairman of the MBTA board, said “passengers will have increased ability to report safety issues to the appropriate personnel.”

The contract also calls for the MBTA to collect about $4 million in fees, an important additional source of revenue. The agency is facing a $16 million deficit this year, which it is addressing by cutting some services.

The MBTA faces an ever larger deficit next year caused by stagnant sales tax revenues, high fuel costs, rising health insurance premiums and an expected drop in advertising fees.

The T tried to install wireless voice and data service in underground areas three years ago, but the company hired to do the job defaulted on the contract because it could not generate deals with wireless companies. The new effort is on a much smaller scale.