Trolleys, subways and 15 key bus routes will begin running until 3 a.m. during the weekends under an MBTA pilot program set to begin next spring, according to a transportation official.

The pilot program is estimated to cost $20 million, and the MBTA hopes to bring on corporate sponsors to assist with the funding of it. The Boston Globe has agreed in principle to be a corporate sponsor, and state transportation officials expect additional sponsors to sign on in the coming weeks.

"Beginning next spring we are going to launch a one-year pilot program, which would launch late-night service on all subway lines, light rail and the 15 key bus routes," a state transportation official told the News Service.

Gov. Deval Patrick had included late-night service in the transportation plan he hoped to fund through $1.9 billion in new taxes, which would have also paid for increased educational spending. The Legislature passed a scaled-down tax package this summer to fund transportation and other state programs.

Unlike the former Night Owl late-night service, which sent buses along train routes, the new pilot will use the subways and trolleys in addition to the 15 bus routes. The last trains will leave the central station at around 2:30 a.m., with service ending at 3 a.m.

Last trains currently leave at around 12:30 a.m. or 1 a.m., well before the last call at many of the bars in the Metro Boston area.

People have long sought longer hours for the T. The MBTA is somewhat hampered in its ability to provide late-night service because there are not redundant tracks, which might allow the trains to continue in operation while regular maintenance is performed.

The late-night fares would be the same as regular fares in the pilot year, and if the program is a success the plan would be to likely increase fares for late-night service in future years. The late-night service would run into early Saturday and Sunday mornings.

Extending the hours of the MBTA, which is overseen by the state Department of Transportation, was a popular issue on the recent campaign for mayor of Boston. Paying for the service has long been a prime obstacle, with the MBTA struggling each year with deficits and budget balancing.

"Expanding and extending MBTA service could benefit the City of Boston but is not something toward which we can direct City revenue," Mayor-elect Marty Walsh said in response to a News Service questionnaire before the preliminary election.

The Night Owl service began in September 2001, and ran until 2005 when budget constraints caused the MBTA to end the program.