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performed on unit SP 9340 between April 27 and April 29, 1989. During this time, several dynamic braking grids and a grid blower were replaced to correct a previously reported dynamic brake defect. According to the chief mechanical officer, based on this extensive work, the unit should have had functioning dynamic brakes during the descent from Hiland.
The results of the train dynamics analyzer tests indicated that in order to replicate the accident sequence, including brake pipe reductions and speed, a train with a trailing tonnage of 8,900 tons would have required the equivalent of three locomotive units with functioning dynamic brakes. Although the Safety Board concludes that when Extra 7551 East began its descent from Hiland, only three of the six locomotive units had functioning dynamic brakes, the Board could not determine, based on the available evidence, whether this total of three units involved the full dynamics of either SP 7549 or SP 9340, or a combination of the two.
After the operating crew of Extra 7551 East picked up their three-unit locomotive consist at the Mojave yard, they determined that one of the locomotive units was not operating. During the movement of the four-unit locomotive consist to pick up the 69 loaded cars of trona, the head-end engineer became aware that the dynamic brakes on one of the locomotive units were functioning only intermittently. When the two-unit locomotive helper consist coupled onto the rear of Extra 7551 East at Oban, the dynamic brakes on only one unit (SP 7443) were functioning. The helper engineer testified that he did not inform either the dispatcher or the head-end engineer because the dynamic brakes on the other unit (SP 8317) were not functioning when he took control of the consist and thus he believed the information had been relayed to the dispatcher by the engineer whom he relieved.
When Extra 7551 East departed Oban, the head-end engineer asked the helper engineer if he had "…all of your dynamics." When the helper engineer responded, "Yeah, I’m in full," the head-end engineer believed that both helper locomotive units had functioning dynamic brakes. Therefore, the head-end engineer believed that he had at least four units with fully functional dynamic brakes. Although the Safety Board is concerned about the lack of communication among the assistant chief dispatcher, the helper engineer, and the head-end engineer regarding the condition of the dynamic brakes on the six locomotive units, the head-end engineer’s belief that he had four units with functioning dynamic brakes was reasonable, under the circumstances.
Trailing Tonnage.—The Lake Minerals Corporation had shipped an average of only 88 tons per rail car when it had intended to ship 100 tons per car on the one previous occasion that it had shipped trona by rail. To avoid a repeat of that situation and also to avoid having excess material at the destination, Lake Minerals requested that the loading contractor at Rosamond install a sensing device on the front-end loader to measure the amount of material that was being loaded into the hopper cars. According to the superintendent of Lake Minerals, the accuracy of the sensing device had been tested and he was confident that each of the 69 hopper cars contained approximately 100 tons of trona. Therefore, the Safety Board concludes that the 69 hopper cars loaded at Rosamond each contained approximately 100 tons
