Donald R. Rackley, 68, of Graham was driving a 1976 Ford pickup pulling a loaded horse trailer north on the interstate at 12:43 p.m. when the trailer hitch broke.
Rackley swerved the pickup to the left, colliding with a 1999 Peterbilt tractor driven by 66-year-old Arnold Horath of Tenino.
The truck stopped in the median on top of the jersey barrier. The horse trailer drifted off into the shoulder of the road.
Rackley was injured and taken to Harborview Medical Center in Seattle. His passenger, Flora L. Woodall, 64, of Graham, was also injured and taken to Providence St. Peter Hospital.
Horath was not injured. The horses were also not injured.
The cause of the crash was defective equipment.