Mechanics for the Los Angeles County Metropolitan Transportation Authority (MTA) went on strike Tuesday, shutting down buses and trains, potentially stranding an estimated 500,000 daily riders of the system. Mechanics walked off the job after midnight and union officials said bus drivers, train operators and other workers would honor the picket lines, halting some 1,900 buses, as well as light-rail and subway lines, said the Associated Press. The decision to proceed with the walkout was made Sunday, after negotiations between the union and the MTA made no progress over the mechanics union's health care fund. The union wants greater contributions from the MTA to cope with soaring medical costs, said the AP. The MTA carries about 500,000 riders a day, which accounts for between 75% to 80% of those in the county who use public transportation, said MTA spokesman Marc Littman. Up to 9,000 union workers could potentially honor the strike.