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PHILADELPHIA (CBS) – SEPTA Transit police are encouraging passengers to follow the rules after a tragedy on the tracks.

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A 7-year-old boy fell to his death while walking between two train cars on the Broad Street Line on Sunday night. His family has identified him as Aden Devlin.

SEPTA officials gave details on their investigation Monday.

“The National Transportation Board has launched an investigation on this incident,” said James Fox, an assistant general manager of SEPTA’s subway system.

Transit officers and Philadelphia police were on the scene investigating on Monday. Each subway connects with three chained links on both sides.

Transit officials say it’s dangerous, which is why there are posted signs above the doors that say, “No passing through.”

SEPTA officials reinforced the importance of riders following the rules.

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“Some folks shrug and say, ‘What’s the big deal?’ This is the big deal. This is the big deal. This is why we do what we do every day to try and keep people safe,” said SEPTA Police Chief Thomas Nestel.

SEPTA police say they enforce the rules daily on the trains for not walking between cars and soliciting, something the boy was doing when he fell.

“The SEPTA family is heartbroken about what happened, the loss of life,” said Nestel. “A 7-year-old’s future ended at a Broad Street subway car and really it pains us. Our thoughts and prayers are with that little boy’s family.”

The chained links design has been used in Philadelphia for at least several decades.

SEPTA subway safety officials discussed a possible upgrade.

“We’re always looking for continuous improvement in our system,” said Fox. “We’re always looking to enhance and make our system better; that means putting a full enclosure and if that’s even doable, that’s something that will have to come out of the investigation.”

Transit police currently give warnings to passengers, not citations.

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The boy was with an adult man at the time, selling candy. That man was not related to the boy.