The Safety Board determines that the derailment was caused by lateral movement of the track under the train. The lateral movement was caused by buckling of the track.
The track buckled because of high compressive forces caused by heat expansion and the tendency of rail to creep in the direction of the predominant flow of traffic and descending grade. The absence of a full ballast section, irregular compaction of the ballast, and reduced density of ballast below crossties resulting from boring operations are possible contributing factors which could not be ruled out. A slight misalignment of the track produced lateral movement of the cars which may have initiated the buckling of the track, which was already under high compressive force.
A number of injuries to passengers were caused by their being thrown from the seats and striking interior parts of the cars and by luggage being thrown about inside cars.