Gautrain services between Rosebank and Sandton stations resumed by mid-morning on Tuesday, the company said.
"We again apologise to those passengers who were inconvenienced this morning as a result of cable theft," spokesman Errol Braithwaite said in a statement.
Earlier on Tuesday commuters were told the Sandton and Rosebank line was closed, and so was the service between Hatfield, Pretoria and Centurion, for the second time in two weeks.
Bus shuttle services ran between Rosebank and Sandton, and Hatfield and Centurion every 20 minutes to transport stranded commuters.
The Democratic Alliance said it was concerned by a second failure, in less than a week, "of the so-called 'world-class' Gautrain".
"The excuse that cable theft is to blame has a hollow ring, especially when one considers that one of the most important questions posed to the department was: could the consortium deal with the high theft rate of copper products in South Africa?" spokesman Neil Campbell said.
In a statement he said that assurances were given that cables would be buried under concrete, but it now seemed that not all cables were 'thief-proof'.
He said the "excuse" of technical difficulties given for the failure of the Sandton and Rosebank line belittled the Gauteng taxpayer.
If existing security measures were incapable of stopping theft, more security should be provided by the consortium. If not, the Gautrain would become "nothing more than exceedingly expensive white elephant".
"We deserve a reliable service after spending so much taxpayer's money, not an additional bus service," Campbell said.
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