Today's story was written by Sharon Fisher. Podcast version here.

Quick – where’s the Trestle Bridge?

Don’t feel bad if you don’t know. It kind of flies under the radar. For a bridge, that is.

Even the people who know where it is aren’t really sure what to call it. Depending on where you look and whom you ask, it could be called the Trestle Bridge, the Red Trestle Bridge, the Oregon Short Line Bridge, the Union Pacific Bridge, the Boise River Greenbelt Truss Bridge, or the Railroad Trestle Bridge. On Google Maps it's called the Boise River Greenbelt Truss Bridge and on Apple Maps it's called the Boise River Greenbelt Trail. Confusing, huh?

Whatever you want to call it, it’s the steel truss bicycle and pedestrian bridge that’s part of the Greenbelt and crosses the Boise River near Garden City. You can see it from the connector while traveling Eastbound into downtown Boise. And this year, it’s celebrating its 100th birthday.

How the Trestle Bridge came to be

Like so much of Idaho’s historic infrastructure, the bridge came by way of the railroad. But the first bridge at that site, built in 1893, was made out of wood.

“The whole point was to access what we think of as downtown Boise,” said Dan Everhart, State Historic Preservation Office outreach historian for the Idaho Historical Society. “The transcontinental railroad didn’t touch Idaho,” instead going through Utah and Nevada, he said. “From 1869 to 1883, Idaho essentially had no rail access.” Instead, the city had to rely on mule teams and wagons to haul freight from Kelton, Utah.

In 1883, the Oregon Short Line was formed, with the intention, as the name implies, of being the shortest route from Granger, Wyoming to Portland, Everhart said. Geography being what it is, that meant the railroad would cross southern Idaho. “Boiseans expected it would come into town and serve the city of Boise,” he said. “The railroad, being the railroad, said, ‘Not so fast.’”

Instead, the closest railroad station to Boise was in Kuna, and the railroad made other stops in Nampa and Caldwell. “They gave the excuse that it was the grade to come off the Bench” that made it too difficult for the train to go into Boise itself, Everhart said. “They’d come over the Rocky Mountains, they’d come over the expanse of southern Idaho, crossing the Snake River, over hills and mountains. To say they couldn’t make it come into Boise was nonsense. It was about the money.”

For example, some cities paid the railroad to stop in their communities, and as you may recall, Boise didn’t do that. In addition, railroads were also granted easements all along their tracks, and railroad companies stood to make a lot of money by platting the land around those stops, Everhart said.

But finally, ten years later, Boise got its rail connection, and a depot at 10th and Front St., and that created the bridge. “From 1893ish to 1923, that was the rail bridge over the Boise River,” Everhart said.

But the rail line was a stub, not part of the main line. People had to come to Boise via Nampa. “The train line eventually went as far as Barber Valley, but it was always a dead-end line,” Everhart said. “There was a roundhouse in downtown Boise to turn the engines around. It was never a through line.”

Finally, in 1925, Boise became part of the main line – at least, to the extent that people didn’t have to go to Nampa first – and that led to an upgrade of all the rail infrastructure in Boise, Everhart said. “That’s why, in 1923, they took out the timber trestle bridge and put in this new steel bridge,” he said. The bridge was built by the Oregon Short Line, which was a subsidiary of Union Pacific, said Patricia LaBounty, curator of the Union Pacific Museum in Council Bluffs, Iowa, in an email message.

Construction of the steel bridge

The bridge would have seen little passenger service as the Boise Depot on the hill, as we know it today, opened in April of 1925, said Eriks Garsvo, director of the Owyhee County Museum and Boise Train Depot tour guide. So it primarily would have been used for freight.

Needless to say, a bridge that’s going to hold up a train and freight, over water, multiple times a day has to be pretty strong. Consequently, the Trestle Bridge was built using the standard of the day, which used Pratt trusses. “It was a pretty common configuration for a steel truss, or even a wooden truss bridge,” Everhart explained. “The whole point of a truss bridge is that it can span longer distances with this ‘Tinker Toy’ assembled structure than a single piece of lumber or steel.”

The triangular structures on the side of the bridge provide a force that holds the bridge up, along with the concrete piers it rests on. And the Trestle Bridge uses a Pratt truss, as opposed to a Howe truss. The Pratt truss was developed by Thomas Willis Pratt (who, incidentally, attended Rensselaer Polytechnic Institute – the same college attended by John Roebling, who designed the Brooklyn Bridge). The Pratt truss was considered to be superior to the Howe truss because, while both of them featured diagonal supports, the Howe truss diagonals faced toward the center, while the Pratt truss faced away. That has the effect of spreading out the load more and can require less steel.

The Trestle Bridge was built by the American Bridge Company, which at the time was the nation’s leading bridge manufacturer, Everhart said. The company went on to build other more well-known projects, such as San Francisco’s Golden Gate Bridge. “This was a relatively small bridge for them as a company,” Everhart said.

The end of the bridge

As automobiles replaced passenger trains, and trucks replaced freight trains, the era of the railroad in Boise was coming to an end. The bridge continued to provide access to the freight lines and warehouse district of Boise until the rail line was abandoned, Everhart said. “The branch line into Boise was scrapped in 1986, and 1985 was the last year of operation,” Garsvo said. Later in the 1980s, the branch line railroad tracks running along Front Street were removed, said Caitlin Hocklander, A&H Program Assistant-History for the Boise City Department of Arts & History, in an email message.

But as in many other regions, the railroad right-of-way and infrastructure gained new life as a recreation pathway, and so too was the case of the Trestle Bridge. Hocklander wasn’t sure how it came to be part of the Greenbelt, but by the early 1990s, the conversion of the old steel railroad bridge into a section of the Greenbelt was complete, she said. While the railroad painted the bridge silver when they maintained it, the city went with the red primer color when it repainted the bridge, Garsvo said.

In 2019, the Trestle Bridge got another boost when three Rotary Clubs, as well as several other organizations, got together to build Rotary Park, a pocket park on the Greenbelt in the area. One aspect of the park is an interpretive sign about the bridge, said John Biggs of Boise Metro Rotary, who spearheaded the project.

What makes the Trestle Bridge special?

The Trestle Bridge was never that particularly noteworthy in its design or construction. “It’s not necessarily notable for its engineering,” said Everhart. “It’s a fairly common engineering type. At one point in time, a steel bridge was a de facto bridge construction technique, especially in the teens and the 1920s. Every bridge, highway or rail, was built as a truss bridge.”

What makes the Trestle Bridge noteworthy now is that these basic ubiquitous steel truss bridges are increasingly rare, Everhart said. They’ve gradually been replaced by bridges that aren’t steel truss, such as by concrete in the 1930s to 1950s.

Moreover, steel truss bridges like the Trestle Bridge stand to become even more rare in the coming few years – not because there’s anything wrong with them per se, but based on the belief that they don’t carry enough weight or that they’re not wide enough, Everhart said. “There’s a huge push under Biden’s infrastructure bills to replace huge numbers of these steel truss bridges,” he said. While these have largely already been replaced on highway networks – “ITD doesn’t have many steel truss bridges left,” said Everhart, a former historian with the Idaho Transportation Department – “there are local and county highway districts that still have them. And they’re being replaced, or funded for replacement, in the next couple of years.”

Other similar bridges in Idaho include the 9th St. Bridge in Boise, also part of the Greenbelt and on the National Register of Historic Places, as well as one in Caldwell, the Caldwell Road Bridge, that will continue to be a bicycle and pedestrian bridge, although a concrete bridge is being built to replace it for automobile traffic, Everhart said. And between Lewiston and Clarkston there’s the Clearwater River Railroad Bridge, a 1930s steel truss bridge that actually goes up and down to accommodate river traffic.

“Idaho has only one automobile bridge that moves, and that’s the one at Lewiston," he said. The State Historic Preservation Office is in the process of listing that bridge on the National Register, he added.

The Trestle Bridge isn’t yet listed on the National Register, but it could be, Everhart said. “It would be eligible,” he said. “The only major change is its use – it’s no longer a rail bridge. But it’s in the same location, and serves a similar purpose, and the materials and design haven’t changed.”

Of course, a National Register listing doesn’t protect a building – or a bridge – from modification or even demolition. But because there’s not as much emphasis in the Biden infrastructure bills on investing in the railroad network, steel truss bridges on rail lines or former rail lines, like the Trestle Bridge, stand the best chance of surviving, especially since it’s now a pedestrian bridge, Everthart said. “It just has to hold up a few hundred pounds’ worth of people, and it could probably do that forever,” he said. “As long as they maintain it and paint it, it should be fine.”

Happy 100th bday, red bridge!

Thanks for reading!

With love from Boise,

Marissa

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This story was written by Sharon Fisher. Sharon is a digital nomad specializing in history and tourism. Check out her book out about the history of Kuna, Idaho. You can read more of her work here.

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