A federal investigation into the death of a child killed while walking between train cars on SEPTA's Broad Street Line will focus on how to prevent such an accident from happening again.

In its preliminary report on the Sept. 23 death, the National Transportation Safety Board wrote Monday that the investigation would look at ways to minimize the dangers associated with the gangways between train cars.

Aden Devlin, 7, of Strawberry Mansion, died when he fell between cars as the train left the Allegheny station platform through a tunnel, according to the NTSB report. Passengers saw the boy fall, the report said, and one activated an emergency alert that led the operator to stop the train.

Aden was killed when he was struck by one of the cars, officials have said.

The boy was selling candy to passengers on the southbound train with his 11-year-old brother and a 26-year-old family friend when he died, police said. Aden had sold candy at 69th Street since he was 4 and started selling on the subway earlier this year, family has said. He had dropped candy on the gangway between cars, his family said, and slipped when he tried to pick it up. His brother and the family friend tried to catch him but couldn't, they said.

>>READ MORE: 7-year-old who died after fall from SEPTA train remembered for entrepreneurial spirit

At the time of the death, SEPTA had signs posted above the end-of-car doors advising riders not to use them to pass through to other cars. SEPTA policy does not bar passengers from using those doors, but the agency highly discourages their use, the NTSB reported.

Since the death, SEPTA has put larger signs on the interior of those doors on all its subway cars and begun making announcements that those doors are to be used only in emergencies.

The gangway between subway cars is protected by three chains on each side. Those weren't enough to prevent Aden from falling onto the tracks.

A SEPTA spokesman said Monday the agency was looking at safety issues related to the gangways but had no specific changes in the works beyond what was mentioned in the NTSB report.

The NTSB will eventually complete a full investigation with findings on how the death happened and recommendations on how to prevent similar incidents.