Lake Minerals had contracted to ship 6,900 tons of trona - a mineral used in glass manufacturing - from the Mojave Desert to the Port of Los Angeles for export to Colombia. The mining company turned in paperwork without filling in weights, assuming the railroad would know that 100-ton hopper cars filled to capacity weighed 100 tons. Clerk Thomas Blair, eyeballing the load, estimated 60 tons per car based on how 100 tons of coal looked. The train's official weight became 6,151 tons. Its actual weight was 8,900 tons. Engineer Frank Holland, conductor Everett Crown, and brakeman Allan Riess picked up their three-unit locomotive set at Mojave at 9 p.m. on May 11. When the lead unit wouldn't start, they grabbed a replacement from another consist and set out for Cajon Pass.

Dynamic brakes work by converting a locomotive's electric motors into generators, using the train's momentum to create resistance. On a 2.2 percent downgrade, one working dynamic brake can control about 1,700 tons at optimal speed. The train needed the equivalent of six functional locomotives to handle 8,900 tons safely. On paper, they had six. In reality, only two worked. The second locomotive was dead-in-tow - no dynamic brakes. The third locomotive's event recorder later showed it produced no braking current, though its cooling fans made sounds that fooled the crew into thinking it was functional. The fourth locomotive's brakes worked sporadically. Of the two helper units on the rear, one also had no dynamic brakes. The helper engineer knew his unit was compromised but never told the head-end crew.

When the train crested Cajon Pass at Hiland and started down the western slope, engineer Holland immediately felt something wrong. The train was gathering speed faster than his inputs could control. He applied air brakes and maxed out his dynamic brakes, then radioed the helper engineer for assistance. The helper engineer, Lawrence Hill, made an emergency brake application - a last-ditch measure that, under federal regulations of the time, automatically disabled all dynamic braking on the train. Now the only stopping power came from air brakes, which work by pressing metal shoes against wheel treads. Air brakes generate tremendous heat at high speed. By the time the train reached Duffy Street, the wheels had expanded so much from heat that they were separating from their axles. The NTSB would observe this twelve hours later. The train hit the 35-mph curve at 110 mph.

The lead locomotives and several freight cars catapulted off the elevated track bed into the residential neighborhood below. Seven houses were demolished. Conductor Crown was crushed in the nose of unit SP 8278. Brakeman Riess died in the cab of SP 7549. Ten-year-old Jason Thompson and seven-year-old Tyson White were killed when a train car destroyed their home. Engineer Holland, still in his seat, suffered cracked ribs and a punctured lung but crawled from his wrecked locomotive. All 69 hopper cars were destroyed. Four of the six locomotives were damaged beyond repair and scrapped on site. Trona - white, powdery, and everywhere - coated the wreckage and the neighborhood. Cleanup crews marked the location of a buried petroleum pipeline with stakes.

On May 25, 1989, at 8:05 a.m., shortly after a train passed through the restored track, the 14-inch Calnev Pipeline ruptured at the exact curve where the derailment had occurred. Earth-moving equipment during cleanup had damaged the line. Gasoline showered the neighborhood and ignited. Flames shot 300 feet into the air and burned for nearly seven hours. The fire killed two more people, destroyed eleven additional houses and 21 cars. Five of the destroyed homes sat directly across from houses demolished in the derailment; one was the only home on the track side of Duffy Street that had survived the crash. Total property damage reached $14.3 million. Southern Pacific subsequently required clerks to assume maximum weight for any car without documented weight. The Federal Railroad Administration reversed its mandate that dynamic braking must disable during emergency stops. Duffy Street's south side was rezoned as open space. The railroad, now operated by Union Pacific, still runs trains over that curve.