SAN FRANCISCO -- An electrical short on an overhead line near Castro Station was the culprit in a San Francisco Municipal Railway breakdown that vexed thousands in San Francisco this afternoon, Muni officials reported Thursday evening.

The short was related to overhead line construction work in the Muni Metro subway being performed by an independent contractor, said San Francisco Municipal Transit Agency spokeswoman Maggie Lynch.

Underground service was restored at 4:30 p.m. Thursday to affected lines -- K, L and M - which had stopped running about 1 p.m.

Lynch apologized for the service disruption in a statement issued Thursday night.

"We are very sorry," she said. "We understand the frustration this sort of incident creates and we are doing everything we can to prevent future problems."

During the disruption, Muni officials deployed 24 buses to provide service between Van Ness and West Portal stations and sent more than 75 staff members to affected stations to assist the public.