Thousands of commuters were stranded Friday morning after a signal failure stopped rail service between Stockholm and the suburb of Södertälje.

Swedish Radion News reports that the electrical failure brought traffic to a standstill about 6:30 a.m. Transportation officials said it's unclear when trains will be up and running again.

"Right now we do not know very much, I've been informed that we have found the cause of the error, but we don't have any prediction," for when it will be fixed, said Beisi Sundin, a spokesperson at the Swedish Transport Administration.

The disruption may cause headaches for other travelers as...