San Bruno B.A.R.T.

  A substation fire at the South Hayward train yard early this morning disrupted BART service between Hayward and Fremont, authorities said.

  The fire, which started at 4:26 a.m., caused electrical outages in the Hayward, South Hayward, Union City and Fremont stations, stranding trains and forcing the agency to roll out bus service between the stops until the system is back online.

  Shortly before noon, the same substation caught fire again, along with a second substation.

  The cause of the fires is unknown, although BART spokesman Linton Johnson said the transit agency is trying to determine if they are related to upgrades that were performed on substation equipment early this morning. No injuries were reported.

  "We're still trying to determine what in the world is going on," Johnson said.

  There are bus bridges currently running between the Fremont and Bay Fair stations at 30 minute intervals. "Right now we're working on bringing more buses in to get the service increased to 15 minutes between those stations," Johnson said.

  While all other BART service is operating on its regular Saturday schedule, service to this evening's radio station fireworks show at the Embarcadero in San Francisco would most likely be affected, Johnson said, because BART will be unable to get longer trains out of the Hayward yard and into the system.