Axles of Dynamic Brakes.— The Safety Board examined the available evidence to determine the actual condition of the dynamic brakes on all six units The head end engineer and the helper engineer were riding in the first unit of the head-end consist, SP 8278, and the last unit of the helper consist, SP 7443, respectively. Their testimony indicates that the dynamic brakes on these two units were functioning. Also, a readout of the event recorder data from unit SP 8278 verifies that the dynamic brakes on that unit were functioning. Although unit SP 7443 was not equipped with an event recorder, the Safety Board believes that the testimony of the helper engineer is sufficient to conclude that the dynamic brakes on that unit were also functioning. The second unit in the head-end consist, SP 7551, was dead-in consist, and the first unit in the hclper consist, SP 8317, while operating in power, had its dynamic brakes cut out and tagged. Based on the physical evidence and the testimony of the two engineers the Safety Board concludes that the dynamic brakes on units SP 8278 and SP 7443 were functioning whereas the dynamic brakes on units SP 7551 and SP 8317 were not functioning when the train began descending the 2.2-percent grade.
The Safety Board received conflicting information regarding the condition of the dynamic brakes on the remaining two units, SP 7549 and SP 9340. The head-end brakeman was riding in the third unit, SP 7549 of the head-end consist. According to the head-end engineer, he asked the head-end brakeman about the condition of the dynamic brakes on that unit, and the head-end brakeman replied, "its revving." According to the SP's chief mechanical officer, even though a unit "revs" in dynamic, one cannot be certain that the dynamic brakes on the unit are actually functioning without checking the ammeter reading in the cab of the locomotive in question. The inquiry by the head-end engineer should have prompted a conscientious brakeman to report any malfunction of the dynamic brakes. The lack of any further comment by the head-end brakeman suggests that either he was not attentive or that the dynamic brakes were functioning. Although there is no evidence to suggest that the head-end brakeman was inattentive, the Safety Board could not rule out that possibility. An engineer's failure report of May 4, 1989, 8 days before the derailment, indicated a dynamic brake failure on SP 7549 because of a stuck motor-braking switch. Although this defect was corrected, the chief mechanical officer testified that this type of defect could easily recur. Therefore, the possibility exists that the motor-braking switch became stuck after the head-end brakeman observed that the brakes were "revving." Data from the event recorder of SP 7549 indicated no amperage in dynamic braking as the train descended the hill. The general road Foreman testified that, based on this information, he believed that the dynamic brakes on unit SP 7549 were not functioning when the train descended the hill. The chief mechanical officer testified, however, that because of past experience with the cartridges from the event recorders not recording accurately, the lack of a recording was not sufficient evidence to conclude that the dynamic brakes were not functioning.
According to the head-end engineer, the dynamic brakes on unit SP 9340 were "intermittent" when he operated the unit from Fleta to Mojave before the locomotives were repositioned for the eastbound trip through the Cajun Pass; that is "it would load and then the dynamics would drop out." Based on a review of worksheets provided by SP, extensive dynamic brake work had been