The service reductions are part of BART's effort to address its estimated $310 million budget deficit over the next four years amid an ongoing decline in ridership, state funding and sales tax revenue.
The reductions were approved by the transit agency's board of directors when the board adopted the 2010 budget on June 11.
The changes mean that BART will return to having train service only every 20 minutes on weekday evenings, weekends and holidays. There's been a 20-minute interval between trains during off-peak hours during most of BART's history, but on Jan. 1, 2008, the transit agency reduced that interval to 15 minutes for the first time.
BART officials said the more frequent service generated only a small increase in ridership so they didn't think it justified the cost.
The other cost-cutting measure is to have only one route serving the San Francisco Peninsula during off-peak hours. Currently, there are two routes during all service hours.
BART spokesman Linton Johnson said today's changes will have at least one positive element because they will restore a direct train connection between the Millbrae and San Francisco International Airport stations during off-peak hours.
He said that was an amenity that BART discontinued in 2008 as a tradeoff to providing more frequent off-peak service.
Johnson also said there will be more even spacing of trains passing through San Francisco during off-peak hours. He said that means there will be fewer instances of trains bound for the same destination running one after the other.
For example, he said, passengers are sometimes frustrated when a train going to Pittsburg-Antioch is followed by another train on the same line instead of by a train going to a different destination, such as Richmond or Fremont.