Carrying 1,900 tons of timber, the cars traveled at about 11 miles per hour past at least 20 public rail crossings, according to a report on the Jan. 23 incident.
The cars came to rest on a curved stretch of tracks, where a surprised Burlington Northern Santa Fe Railway crew found them minutes later in the path of their freight train.
"I just went, wow, wow, wow," Richard Wright, Union Pacific's local yard manager, told an investigator later, according to a 340-page transcript of the railroad's internal investigation obtained by The Oregonian and reported in today's editions of the newspaper.
At the heart of the incident lies a relatively new technology being used in railyards nationwide: remotely controlled switch-engines. A remote, as the systems are called, pushes train cars around a site without requiring an engineer in the locomotive, and probably propelled the unmanned railcars out of the Portland railyard in January.
The remotes normally are operated by a crew of two - a foreman and switchman - instead of a three-member crew led by a train engineer.
One or both workers wear a "beltpack" the size of a toaster that sends signals to a centrally located computer, which then signals a receiver in the locomotive. Safety features built into the Beltpack are designed to halt the engine if the operator falls over or if communication is somehow blocked, Canadian National officials say.
Though no one was injured, the incident in Portland stoked a long-simmering dispute between union members and Union Pacific, the nation's largest railroad, about use of the remotes.
Officials with the Brotherhood of Locomotive Engineers and Trainmen say the remotes are unsafe and are contributing to a service slowdown across Union Pacific's 33,000-mile system. They say the incident raises concerns about the thoroughness of worker training as the railroad rushes to replace a rash of early retirees and accommodate a surge in business.
The worker at the controls during the January incident had been certified to operate a remote engine for only five days and had never before worked in that yard, according to the transcript of the subsequent investigation.
In February, Union Pacific fired the crew operating the remote that evening - both members of a conductor's union - citing negligence and rules violations, including failure to apply a "sufficient number of hand brakes" to prevent unattended cars from moving.
Both workers appealed and recently accepted a settlement returning them to work.
The runaways damaged several switches, costing the railroad $10,000 in repairs. But managers were more alarmed at the risk they posed to the public. If line switches had been set differently, authorities say, the rail cars could have rolled across a bridge and through Amtrak's passenger terminal in northwest Portland.