Alaska Railroad officials blame water-softened roadbed for the derailment Monday of five engines and 11 cars of a freight train near Houston.  The repair crews on the scene think it was a flash flood in the mountains that released a surge of water into the swollen Little Susitna River that caused the wreck.  One engine unit was righted and towed to Houston Tuesday afternoon.  Crews are presently at work building a new roadbed bypassing the wreck and the rail link is expected to be open tonight or tomorrow.