The reason that happened was because telegraph lines were hundreds of miles long without interruption, so otherwise tiny charges could build up with distance.
Several years ago, there was a study done on exactly this sort of event. It concluded that the power grid was in severe danger from it. Those lines stretch for many miles “uninterupted”. It concluded that the most vulnerable part were the huge transformers at either end, which were likely to be destroyed, and which would put out power for about 70% of the population.