As of about 8:15 a.m. minor delays were reported but train service was nearly back to normal.
The problem was reported at about 4:45 a.m. Johnson said a panel had fallen off an overnight rail grinder, a vehicle that travels along the tracks to essentially smooth any rough patches to cut down on noise.
A Millbrae-bound train struck the panel, causing damage to the third rail, Johnson said.
The damage prevented trains from using the westbound tracks, so BART was forced to single-track trains through the tunnel on the eastbound side.
The situation created delays of 30 to 40 minutes.
Johnson said the temporary repairs will allow trains to operate normally today but that additional repairs will be made overnight.