Standing tall within the clouds, the smokestack atop the old Pennsylvania Railroad steam plant near Amtrak's 30th Street Station was imploded yesterday morning with a thunderous boom.

Right on schedule at 7:45, the smokestack, which extended 323 feet off a six-story base into the city's skyline, leaned forward until it crashed to the ground, leaving puffs of dust.

The planned dynamite implosion and demolition of the crumbling smokestack was completed in seconds.

A few passersby captured the moment with their camera phones.

"It's a real historic event," said Tiffany Dow, 26, standing outside 30th Street Station, holding a thermos of coffee. Dow, a Chicago native, works as an administrative assistant in the Cira Centre near the old steam plant. "I just wanted to see it. It's something that holds a lot of meaning," she said.

The smokestack - the Penn Coach Yard Chimney - was built by the Pennsylvania Railroad in 1929. In its heyday, the octagonal relic churned out the steam and electricity that once kept the rails moving.

That technology allowed the train company to electrify its fleet and helped Market Street to blossom with businesses and high-rise office buildings.

Such accomplishments earned the steam plant, closed in 1964, a place in the National Register of Historic Places. The smokestack once was one of West Philadelphia's tallest structures.

For the implosion, traffic through SEPTA, Amtrak, and nearby streets was temporarily halted. Philadelphia and Amtrak police set up a safety zone around the chimney, and air-quality experts were on site.

The demolition of the steam plant itself and the adjacent building, a former Pullman dormitory for the mostly African American railroad attendants, is expected to be completed by the end of the year.

Earlier this year, the former Youth Study Center, located on the Benjamin Franklin Parkway between 20th and 21st Streets, was demolished to make way for the Barnes Foundation, which owns one of the world's largest collections of Impressionist art.

There are no immediate plans to redevelop the old steam plant's site.

Contact staff writer Kia Gregory at 215-854-2601 or kgregory@phillynews.com.