The early morning outage led to some delays in train traffic, said Jesper Ekenlund, a spokesman for power company Fortum. Nearby hotels and shops also were affected, he said.
"The rat had sneaked into a secondary substation and came into contact with some parts that caused it to short circuit," he said.
"It must have been really big because there's a certain distance between the parts it touched."
Ekenlund said Fortum will now have to decontaminate the area where the rat met its fate.