derailment site from the west, the track grade descended between 2.0 and 2.2 percent for 22 miles before it transitioned to a O.O-percent grade at the derailment site. In the 22 miles of descending grade, there were 56 curves which varied in degree of curvature from a maximum of 6 degrees to a minimum of 30 minutes.
The track was constructed of 119-pound continuous welded rail (CWR) on tangent track and 136-pound CWR on most of the curves. The 4-degree curve at the derailment site was laid with new 136-pound CWR in 1986. The rails rested on double shouldered tie plates and 9-foot hardwood. crossties and were restrained with two rail-holding spikes on the gage side of the rail, one rail-holding spike on the field side of the rail, and one plate-holding spike on each side of the rail. The rail was box-anchored every other tie. The ties were laid in a ballast of crushed rock.
The 4-degree right-hand curve (based on the direction of movement of Extra 7551 East) at the derailment site was constructed on a fill (levee) with a maximum height of about 21 feet. The curve was 2,474 feet in lenqth, including a 376-foot spiral on each end, and had a 1-inch superelevation.
According to SP Timetable No. 2, the authorized maximum timetable speed for the curve was 30 mph. The Federal Railroad Administration allows a maximum operating speed of 38 mph for a 4-degree curve with a 1-inch superelevation.
About 680 feet of track were destroyed during the derailment. Because of the extensive track damage, there were no distinguishable marks to indicate the point of derailment.
Signals.—Trains on the single mainline track are governed by a traffic control system using colored lights on wayside signals. An inspection of the signal equipment in the area of the derailment was conducted on May 13, 1989. The inspection revealed no problems with the signal system.
At the time of the accident, Extra 7551 East consisted of, from front to rear, 4 road locomotive units (SP 8278, SP 7551, SP 7549, and SP 9340), 69 open-top hopper cars loaded with trona, and 2 helper locomotive units (SP 8317 and SP 7443).
Locomotive Units.—All of the locomotive units were manufactured by the Electro Motive Division (EMD) of General Motors Corporation. These units were six-axle, SD models with 26L automatic brake valves and extended range dynamic brakes.
- With extended range dynamic brakes, as compared to standard range, more retarding force is available from 6 mph up to a speed between 18 and 25 mph depending on the gear ratio.