OAKLAND -- Union Pacific crews were working Tuesday to get four freight cars back on the tracks after they derailed at an Oakland train yard the night before.

No one was injured during the incident that took place about 7:10 p.m. Monday near 13th and 14th avenues, parallel to BART tracks, said Mark Davis, a spokesman for Union Pacific.

The derailment affected four boxcars -- one containing frozen meat and three carrying lumber, Davis said.

"They were sorting them by destination ... moving them around when one tipped," Davis said. "With the other three, the wheels just came off the rail. We're in the process of putting them back on the rail now."

The cause of the derailment was under investigation on Tuesday.

As of 9:30 p.m. more than a dozen workers were on scene attempting to get the four boxcars back on the track. Davis estimated they would be done by about noon on Tuesday.

Harry Harris contributed to this report. Contact Natalie Neysa Alund at 510-293-2469. Follow her at Twitter.com/nataliealund.