A busy passenger and freight rail line was shut down for nearly five hours on Thursday after police investigating the death of a woman struck by a train in Seattle said they found a possible explosive near the tracks.

Authorities later determined the woman had nothing to do with the device, which resembled a pipe bomb, Burlington Northern Santa Fe railway spokesman Gus Melonas said.

It was determined by experts that the subject was not connected with the device, Melonas told Reuters.

He said experts now believed the object found near the woman's body to be a firework-type device, although it was still being examined.

The incident began at about 9 a.m. local time when the woman, who was not identified, was struck near a Seattle park by a 63-car Burlington Northern freight train headed to Tacoma, Washington from Chicago, Melonas said.

Melonas said train operators who saw the woman sounded the locomotive's horn and went into an emergency braking operation but were unable to stop in time.

The subject appeared to be walking away from the rail line and for some reason knelt down at the last moment, Melonas said.

Police who responded to the scene found the device, prompting a shut down of the rail line used by 40 trains daily and which serves as a main connection to Canada.

Melonas said three freight trains and one Amtrak passenger train were delayed before the line was reopened shortly before 2 p.m. local time.

The woman's death marked the fourth fatality on Seattle-area tracks since August 5, Melonas said.