WASHINGTON — The engineer driving an Amtrak train wasn’t using his cellphone to talk, text or download data just before the train derailed in Philadelphia last month, safety investigators said Wednesday, deepening the mystery of what caused the accident that killed eight and injured about 200. In an updated report, the National Transportation Safety Board also said the engineer, Brandon Bostian, didn’t access the train’s Wi-Fi system while he was operating the locomotive. Investigators have said previously that the train accelerated to 106 mph in the last minute before entering a curve where the speed limit is 50 mph.