The derailment was reported at about 12:30 p.m. in the area of Cornwall Street, said Randy Sawyer, the county's chief environmental health and hazardous materials officer.
Sawyer said the train car that derailed was carrying waste from the Tesoro refinery that included sodium sulfide and sodium hydroxide.
He said the mix is "very corrosive and very caustic," but that residents are not believed to be in danger.
"It doesn't look to be that major at this time and we don't expect any off-site impacts, but there may be road closures in the area," Sawyer said.
He said no shelter-in-place order has been issued.