David Bergstrom was returning home from a grocery run Tuesday morning in St. Paul when an unusual sight struck him: a 7-year-old boy sitting on top of a large garbage truck -- in traffic.

"Why is he up there?" Bergstrom thought to himself. "I should do something."

The boy was wearing a T-shirt in the chilly weather, and had apparently run away about 10:18 a.m. from his nearby school, RiverEast Elementary and Secondary School at 1845 Sheridan Av. Bergstrom tailed the truck starting at the corner of St. Paul Avenue and Davern Street in the Highland Park neighborhood.

He pulled alongside the truck at a stoplight and rolled down his window, waving and yelling.

"Stop! Look up!"

Bergstrom pointed at the boy, but couldn't get the driver's attention. The light turned green, and Bergstrom pulled ahead of the truck, waving his arms out his window.

"I knew the kid was in danger and we had to get [the truck] stopped," Bergstrom said.

The boy was crouched toward the rear of the truck, holding onto the rails of a ladder that led up the back.

Perplexed, the truck driver pulled over in the 1200 block of St. Paul Avenue, where he exited and discovered the boy, Bergstrom said.

John Uhl was getting ready for work when the truck stopped in front of his home.

"I was just amazed that the kid was on top of the truck," Uhl said.

Uhl and a neighbor ran out as the truck driver carried the boy down. He seemed OK, but his nose was runny and his hands were bright red from the cold, said Uhl and Bergstrom.

"He was actually very calm," Uhl said. "I'm just happy everything worked out the way it did."

"I don't think it really fazed him," said Bergstrom, a retired pipe insulator who was born and raised in the area.

The boy waited inside a neighbor's house until police arrived, and was returned to school about 11 a.m., said Sgt. Paul Paulos, a police spokesman.

St. Paul Public Schools spokeswoman Toya Stewart Downey said school administrators immediately reported the boy missing and also notified his parents. The child, who has a mental disability, is outfitted with a GPS tracking device used by police to find vulnerable children or adults in the Project Lifesaver program, Paulos said.

The department has 35 clients in Project Lifesaver, 34 of them children. Fifteen officers are trained to use the technology, Paulos said.

Police had arrived at the school and were beginning to track the GPS device when a passerby notified them that the boy was playing on nearby rail cars off Edgcumbe Road. Shortly afterward, police received information that a driver had seen the boy atop the truck.

The school is about half a mile from where the truck pulled over.

"It just goes to show you how when people pay attention and they don't turn a blind eye ... a good outcome can come from a possibly tragic situation," Paulos said. "He's very fortunate."

Chao Xiong • 612-270-4708 Twitter: @ChaoStrib