Late-night or round-the-clock BART service, an idea that's been briefly tested over the years but never adopted, could get a six-month tryout beginning this fall.

Under a demonstration project that will be presented to the transit agency's Board of Directors on Thursday, trains that now depart the end-of-the-line stations at midnight would leave an hour later on Friday nights. But to protect the amount of down time needed for maintaining the system, Saturday service would begin an hour later - at 7 a.m.

"People have always been clamoring for late-night service," said Bob Franklin, board president, who has supported the idea of running BART into the early morning. "We should see if the demand is there."

If BART directors back the idea for late-night service, the demonstration project could be included in the 2012 fiscal year budget - and trains could start running later on Fridays in September.

How it would work

The demonstration project proposes running two additional trains, 30 minutes apart, on each line, with the last one departing an hour later than they do now. The trains would serve all 44 stations.

BART riders have long campaigned for late-night service, saying that the midnight closing time is too early for those who want to enjoy San Francisco's nightlife, get off work late, or arrive at the San Francisco or Oakland airports after the witching hour. But the cry for later service has picked up in recent years. A Facebook page called "Make BART trains run 24 hours" is "liked" by more than 23,000 people.

BART has tested 24-hour and late-night service before, but it never stuck. For years, BART has run all-night or very late service on New Year's; it ran round-the-clock for a month when the Bay Bridge was closed after the 1989 Loma Prieta earthquake and when the bridge was closed for construction and emergency eyebar repairs in 2009. From April to July in 1993. BART stayed open two hours later on Fridays and Saturdays, offering service every 20 minutes between Powell, Embarcadero, MacArthur and Orinda stations in a promotion it called "the Spring Surprise." Only about 200 people took those trains.

BART officials estimate, based on ridership during the recent Bay Bridge closures, that about 2,600 people will ride the system during the extra hour. But they also guess that about 75 percent of the 3,900 people who ride BART between 6 and 7 a.m. on Saturday will end up staying home or finding another way to their destination.

Revenue loss

Paul Oversier, BART's assistant general manager of operations, said the estimated revenue loss of running regular late-night Friday service would be about $110,000. The overall cost of running the service would be $1.2 million for the demonstration project, including startup costs. It would cost about that much to keep the service for an entire year.

But the main drawback, Oversier and Franklin agreed, would be the impact on Saturday's early-morning riders.

Before the BART board could proceed with even a test project, it would have to study the effect on its Saturday early birds.

"We need to find out who they are and how they would be affected," Franklin said.

BART needs to start an hour later on Saturday if it runs an hour later on Friday to maintain the three-hour, 15-minute period it has to work on the system overnight, Oversier said. BART's weekday schedules leave just an hour each night for routine and minor maintenance, he said. The longer periods between Friday and Saturday, and Saturday and Sunday, when four hours and 45 minutes are available, are required for major work, including rail replacement, electrical work and changes to the train control system.

The previous forays into late-night service have required deferring maintenance, Oversier said.

"The 13 hours a week we have is barely enough," he said. "We need every minute of every hour."