The woman, identified as a cleaner in her early 20s, Tuesday took the train from its depot on the Saltsjobaden commuter line that links Stockholm to its upscale suburb. The train broke through the end-stop barrier and careened into the building. No one in the building was injured.
"It's priority No. 1 for our investigators to interview this person," police spokesman Lars Bystrom told the TT news agency Wednesday.
The woman was taken to a hospital where doctors said she sustained serious but not life-threatening injuries.
A motive for taking the train was not known, TT said. Police said they also want to talk to about how she took the train.
Police official Anders Lindstrom said during a news conference a key was in the train and "the brakes weren't on." To get a train moving, the driver must override a safety feature.
Police also are looking into how to safely remove the wreckage that remained in the two-floor apartment building.
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