The County Street MBTA commuter parking lot was closed temporarily Monday morning when fire erupted in the hopper car of a freight train carrying a load of trash. Mansfield firefighters responded to the blaze around 6 a.m. and used a tower truck to spray a stream of water inside the car. Fire officials said it was the third such occurrence involving a CSX freight train this year. The parking lot, used mainly by commuters in the Foxboro area, was reopened shortly after 7. Foxboro Fire Chief Roger Hatfield said he and Mansfield Fire Chief Neal Boldrighini plan to have discussions with the railroad and the shipper who loads the cars with trash in an attempt to avoid such fires in the future. Fire officials theorize contents may be sticking up over the top of the car and contacting the high voltage wire that powers Northeast Corridor Amtrak passenger trains, sparking the fires.