Billowing flames and smoke poured from the diesel-powered articulated bus, near the 85th Street exit, just after it broke out at about 7 a.m.
Sound Transit later said Friday's frigid temperatures and a frozen brake line could be to blame for the blaze.
Officials said 24 passengers were aboard the bus when the fire broke out, including one passenger in a wheelchair.
Fire Department spokesman Kyle Moore says the driver realized something was wrong underneath the bus and pulled over onto the shoulder. He checked the exterior of the bus, then called for help after spotting flames, opened both bus doors and got everyone off, including the passenger in the wheelchair.
"It was scary because it takes some time to unhook those straps, but later I realized she was already free, so people carried her out ... everybody's running away," said the driver, who asked not to be identified.
The driver said he pulled over after several motorists passing by him pointed at the bus to let him know something was wrong. He checked the bus and spotted a small fire near the back.
"At first I thought it was small, and it can be handled," he said. "But then I called in, and it was already spreading up, so I opened both doors and told people, 'We have to run.'"
The driver and some passengers told KOMO News that they heard a pop or explosion.
"I said, 'We have to - we have to run,' because after the first explosion there was fire, so I told them, 'We must run,'" recounted the driver.
"Everyone starts looking around at each other, like, 'Whats happening?'" said passenger Emalasa Franulovich. "Then we hear a second 'pop,' and everyone starts panicking and yelling, 'Get off the bus.'"
"So we're walking down the express lanes watching the bus on fire. ... I'm happy that I'm alive - it wasn't very fun," she said.
The driver said that the evacuation of the bus went very smoothly once it started.
"They were actually very organized, so I told them ... 'Get against the barrier,' and they did," he says.
Some passengers called the bus driver a "hero" for his calm but effective actions in getting the bus evacuated safely.
Sound transit spokeswoman Kimberly Reason says the cause of the fire is under investigation but it could have been caused by frozen brakes that overheated.
The blaze caused major disruptions during the morning rush hour. Aerial footage showed a huge backup extending for miles to the north.
All express lanes were closed initially, as well as two northbound lanes as firefighters battle the flames. The north 85th Street overpass also was closed so that fire trucks can reach the bus from above.
Firefighters were able to extinguish the blaze by about 7:30 a.m., but their efforts were hampered at one point by a shortage of water.
By 7:50 a.m., one lane of the express lanes was reopened and a second was opened at about 9:30 a.m., but traffic was delayed through most of the morning.
Another bus was sent to pick up passengers evacuated from the burning vehicle.