LA's Metropolitan Transit Authority operates most of the county's bus and rail lines. Its contract with unions that represent about 6,800 drivers, mechanics and clerks expired at 12:01 this morning.

Though union officials didn't order a strike, a negotiator said late Monday night that a walkout is still "a very real possibility" if more progress in the contract talks isn't made soon. Unions here traditionally have provided a 72-hour notice before a strike begins.

MTA officials are asking the union to accept a four-day workweek to reduce overtime pay. They also want changes in work rules that sometimes require highly paid mechanics to perform work that can be done by lower-paid employees.