The open-gangway system is popular in Europe. In America, not so much.

A 7-year-old boy died Sunday night after he slipped from a moving SEPTA train onto the Broad Street Line tracks. The boy had reportedly been selling candy, and was crossing between two cars during travel between the Allegheny and Lehigh stops when he fell.

The boy, whose name has not yet been released, was said to have been with his 11-year-old brother and a 26-year-old man when he died around 6 p.m on Sept. 23. Police investigations are ongoing as local officials try to uncover exactly what happened. The National Transportation Safety Board has also launched a federal investigation.

“This is a horrible, horrible thing,” said SEPTA Police Chief Thomas Nestel. “When you imagine a 7-year-old boy on our subway cars, you picture him sitting in the seat.”

But it’s possible the accident could have been prevented entirely. What if, when the young boy was crossing between cars, he hadn’t needed to exit the train at all?

A design that allows protected car-hopping does exist. In fact, it’s used by most European public transportation systems (and by one in the United States). Called the open-gangway system, the model has been found to reduce risk of falls significantly.

But like many novel ideas, American transit agencies have been reluctant to implement it.

Safer, roomier…and more profitable

There are signs posted on every SEPTA train warning people not to pass between cars, according to James Fox, SEPTA’s assistant general manager for system safety. The pathway that exists isn’t meant to be used while the train is in motion. In the official view, it exists only for emergencies.

Fox wouldn’t say how often SEPTA sees accidents — injuries or fatalities — among people passing between subway cars.

But that doesn’t mean passengers follow the rules. Children selling candy and groups of people dancing hop from car to car on the regular. And a system under which this behavior wouldn’t have the chance to turn deadly does exist.

When it comes to the pathways between train cars, there are essentially two design options: open gangway and closed gangway.

SEPTA train cars boast the less modern of the two (shocker). Philadelphia’s transit authority uses closed-gangway rolling stock to connect cars on the Broad Street, Market Frankford and Regional Rail lines. The closed-gangway system is also sometimes referred to as a centipede train.

In that model, all train cars are connected by a metal walkway featuring three to four chains on either side. To access the pathway, you must exit the car you’re currently on by opening a door.

Contrast that with the open gangway, which is widely considered more modern, efficient and safe. In this model, trains are connected by an accordion-style exterior. On the inside, passengers can freely pass through cars inside a comfortable, closed walkway that blends in with the regular interior of the train.

With open gangways, people can walk from one end of the train to the other without stepping outside the train.

Research on the open model has shown the following benefits:

More cash for the transit agency (a train set up this way can fit more passengers, which means more chance to collect fares)

More comfortable rides (passengers can evenly distribute themselves among cars without risking their lives)

More safety (it’s pretty much impossible to fall into the train tracks when switching cars)

SEPTA says its cars are ‘typical’

By 2009, 75 percent of transit agencies outside the U.S. had already adopted open-gangway technology. Meanwhile, in the U.S., only Honolulu’s transit authority has fully implemented this system. New York City is working on it.

Is Philadelphia next? SEPTA officials were tight-lipped about that possibility at a Monday afternoon press conference.

Basically, officials said next steps depend on the results of the federal investigation.

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

Meanwhile, Fox asserted that the current SEPTA model is standard in the industry — which is true if you only consider American subway cars.

“As of now, we consider the car to be pretty typical for the industry,” Fox said, “and not designed or intended to be walking through when the car is in motion.”