Traffic Lab is a Seattle Times project that digs into the region’s transportation issues to explore the policies and politics that determine how we get around and how billions of dollars in public money are spent.

By now, a few million people have cruised across Lake Washington on light rail, enjoying the views, the easy Seattle-Eastside connection and an escape from traffic.

When trains as hefty as 600,000 pounds glide over water at 55 mph, the journey might be taken for granted.

But this is the world’s only passenger train on a floating bridge, a structure that shifts ceaselessly in all directions, as winds and waves nudge the mostly hollow pontoons below.

The region’s engineers, politicians and even voters took a leap of faith to launch this project without any guarantee of success. It became a history-making monument to invention.

“We were trying something for the first time, and we needed to make it work,” retired project director Ron Lewis says in a video retrospective.

Failure would mean squandering at least a billion dollars, and starting from scratch on some other kind of crossing. Years of thought went into new technologies that help the bridge withstand more weight, and preventing the worst nightmare: a train full of people derailing into water.

“It’s sort of Sound Transit’s moon shot,” John Sleavin, a senior technical director, said during construction.

Sound Transit overcame a geologic challenge that dates back 16,000 years to when the Canadian ice sheet retreated, carving Puget Sound and Lake Washington.

The lake bottom is 200 feet deep and too silty to plant bridge columns. Native people and eventually settlers crossed by canoes, small ferries and whaling ships. By the early 20th century, engineer Homer M. Hadley realized the water itself could support concrete pontoons, and therefore a huge road. Washington state opened its first floating highway in 1940, now known as Interstate 90.

Fast forward to this spring, when opening-day crowds filled the railcars, running in converted I-90 express lanes. Light rail ridership systemwide has reached 175,000 passengers per day, a boost of almost 50,000 boardings. There’s been one breakdown, on May 4 when a train stalled for 45 minutes next to Mercer Island after a voltage spike near Federal Way.

The I-90 segment opened six years late: Bellevue route disputes took three years, and a rebuild of track ties Sound Transit deemed defective added three years more. Nonetheless, the 7-mile corridor for $1 billion is far cheaper than a new train-only bridge or tunnel.

In the end, engineers and hard-hat workers solved three puzzles:

• Strengthening the bridge to withstand heavy trains.
• Inventing and installing tracks designed to make derailments impossible.
• Controlling electric current, so stray current won’t ruin track ties, pontoons and underwater anchor cables.

One night in 2005 — three years before voters passed higher sales taxes for more rail — a caravan of flatbed trucks paraded back and forth, laden with steel beams, so the Washington State Department of Transportation could prove I-90’s westbound Homer Hadley Bridge could support a four-car train going each direction.

Proving the bridge could stay afloat, however, was just the first mission.

WSDOT demanded a rail design that preserves the bridge for at least 70 more years, and named two dozen technical risks Sound Transit needed to troubleshoot before the state would allow I-90 trains.

Contractors cinched the middle 10 of the bridge’s 18 pontoons into a single rigid barge, so individual pontoons don’t bob haphazardly when trains pass. Workers strung 1 million feet of steel cables within cramped pontoon cells, a task they compared to “building a ship in a bottle.” Extreme jacking force at either end, a process known as posttensioning, compressed the concrete so tight the bridge shrank 3 inches.

This united structure must stay buoyant. Taking lessons from the truck caravan, engineers calculated a stalled train midlake could sink the bridge 8 inches in a bad-case scenario. And since even intermittent plunging would harm pontoon anchor cables that restrain I-90 from drifting, the team strove to keep that motion minimal.

Meanwhile, the so-called dead load increased by 3,200 tons of new rails and ties (10 times the train weight). Contractors offset that somewhat by scooping out some gravel ballast from the pontoons and removing some concrete highway barrier. Lightweight concrete was ordered for 9,000 prefabricated blocks that support the rails. A custom-blended epoxy secures those blocks to the bridge deck so there’s no need to perforate it with thousands of bolts.

What were the results?

On opening day, the first train, packed with dignitaries and rail fans, landed at the first pontoon 53 seconds after leaving Seattle’s Judkins Park Station, as if your family stepped off a marina gangway onto a tethered dock. That March 28 load pushed the bridge down a skosh more than 1 inch, further proof it’s stable, based on University of Washington live data.

Train service will rarely be disrupted by harsh weather, because prevailing winds blow from the south. The eastbound I-90 bridge shields the westbound bridge where trains go. To cancel trains, waves would have to be 2.2 feet or higher, predicted to happen once per decade, if winds exceed 40 mph for 71 minutes.

A stiff north wind would limit the bridge to one train at a time, expected twice per year on average, if 1.5-foot waves pound the bridge. (This happened for three hours on March 11, slowing a few practice trains.)

Light rail really isn’t crossing one bridge, but three kinds of spans: viaducts over solid ground, sloped transition spans that hinge from land to lake and pontoons afloat.

Given that lake heights fluctuate 2 feet over the course of a year, while waves, wind and train momentum jostle the bridge, how do trains stay steady?

In the run-up to the 2008 campaign, for voters to approve Eastside, Lynnwood and Federal Way rail funding, Sound Transit staff assured a panel of experts that floating-bridge trains would be safe, but admitted they might be slow. There was no design yet.

At first, the team recommended track joints on springs, with adjacent rails that slip between each other, giving and taking slack when the bridge shifts.

Fortunately, a British engineer named Andy Foan invented a more resilient kind of connection for Sound Transit, in which 43-foot-long platforms carry tracks over the hinge points.

Instead of multiple rails that rendezvous and slip-slide, the rails are welded to stay continuous across the lake. They’re affixed to rows of wafer-shaped seismic bearings, much like those that support roofs at Lumen Field and T-Mobile Park. The bearings flex to match bridge motions, while the rails stay stout, uninterrupted and parallel. 

“We’re not worthy!” Sleavin thought, when Foan arranged a model from coffee stir sticks. 

Experts proclaimed it “a brilliant solution” and “a man-made wonder.” Prototypes passed tests at a Colorado research yard and UW labs.

The interconnected system distributes force and motion, leading to “very smooth, imperceptible bending in the rail that is spread out,” said Matthew Barber, bridge designer for engineering consultant WSP. To passengers, it feels like any other straight railway.

That stability is the main safeguard against derailment. In addition, steel guardrails were built between the tracks to restrain train wheels if they ever slip out of place.

If not for those eight special track platforms, “It probably would have been impossible to put trains across the bridge,” Sleavin said in an “Impossible Engineering” episode. “At the very least, we would have had to stop the trains, and almost just bounce the trains across.”

Here’s the amazing part. About 80% of rail movements are produced by lake and bridge motion, while those 300-ton trains impart only 20%, said Craig DeLalla, director of asset-management engineering.

(Car lanes don’t need so much protection from bridge motion, since rubber tires and tiny adjustments by drivers compensate, making the effect negligible.)

Once trains level out at the lake surface, their routine wheel vibrations get absorbed by a cork-infused membrane between the track support blocks and the pontoon decks. 

“That’s why everyone’s saying, so far, that as you go onto the bridge, it’s as smooth as butter — because you’re on all of these different layers of flexible material,” said UW assistant professor Travis Thonstad.

America’s first electric streetcars wreaked “destructive action” by spilling stray current into the ground, ruining city utilities, as probed in the classic 1897 manual “Electric Railways and Tramways” by Philip Dawson. Boston telephone cables were the first to disintegrate. Then in New York, iron water pipes in Brooklyn crumbled one month after a new train route opened. 

Scientists soon invented copper conduits that coaxed electrons from rail to rail, avoiding the ground. Ceramic insulating parts, installed quickly by work gangs, further reduced the waste of electricity.

Like the streetcar builders of yore, engineers for the Seattle-Bellevue 2 Line worried about whether stray current may cause corrosion here. 

After powering the trains, electricity should return through the rails to a Sound Transit substation, and not leak into I-90, so it can survive until the next century.

To further complicate the job, water conducts electricity, and plenty of rain sprinkles here. Runoff that seeps into concrete may eventually cause rebar to rust and bloat, then fracture pontoon walls from within. 

Some 9,000 track-support blocks on pontoon decks were swaddled in waterproof rubber, and small flumes whisk runoff away like a household gutter. As a backstop, a grayish rubberized coating was painted onto the bridge deck to further disperse any runoff. 

If all else fails, long collector wires are attached to rebar within the blocks, so this essential steel won’t rust from fugitive current.

Meanwhile, bridge technicians continually manage electric flows at large scale, a mission called cathodic protection, as a whole network across the lake.

The honeycomb-shaped pontoons contain 287 devices where electricity can be fine-tuned and steered, rather than penetrate into structural steel.

“Think of charging 287 different iPhones to exactly 81% while the iPhones are floating,” said spokesperson Henry Bendon.

Hundreds of metal anodes dangle off the bridge, submerged in water, where they protect the I-90 anchor cables. When pulled out of water, the anode leashes look like Indiana Jones’ bullwhip. The anodes contain “sacrificial” metal that’s supposed to disintegrate.

“These cathodic protection systems are basically allowing you to choose the thing that’s going to corrode,” explained Thonstad.

Such a sophisticated bridge receives constant pampering.

Besides a full-time WSDOT maintenance team, frequent inspections and occasional underwater exams, UW and local tech companies built a “digital twin” that replicates the bridge components on computer screens, while blasting data about the bridge’s behavior — like that reassuring 1-inch pontoon dip on opening day.

Sensors in the lake record water pressure to provide an indirect measure of bridge buoyancy, at 0.1-inch precision. “As the pontoon goes down into the water, there’s more water above the sensor,” Thonstad explained. 

Anchor-cable stresses are reported each minute. Bridge positions are updated hourly by GPS, providing a window into weather effects.

Live feeds verify that the customized track platforms are 100% aligned. They will also help technicians decide where to maintain and adjust rail and pontoon equipment.

Someday, the digital twin will take the next leap to allow remote control of intricate parts, Thonstad said, just as NASA does when fine-tuning its Mars rovers.

While bridge guardians nosh on mountains of feedback, travelers are at liberty to focus outward and savor the Northwest’s ancient glaciers and volcanoes. 

Mount Rainier might even be out.