I have been told different stories about what this wall is, but what I think is most plausible, given it's position and the orientation of the surrounding land, is that this is a retaining wall built to allow easy transfer of sugar cane from trucks to cars of the train. I think that the trucks drove up on the east side of the wall (where the ground slopes up to the top) and the train ran on tracks at the base of the wall. This would have made it easy to load the train.

The Japanese developed a large sugar industry here, with Tinian and Saipan together producing a very large percentage of the sugar produced in all of Asia.

Post-posting, I was also informed that there is a widely-held theory that this was part of the CIA training facility of Northern Saipan. For many years, prior to the U.S. officially entering the Vietnam War, the CIA used the northern half of Saipan to secretly train insurgents, not only to infiltrate Vietnam, but also to act against China and other countries we did not agree with. This wall is within the area which was then controlled by the CIA. As I have heard more information, I have come more to accept this theory as likely.

If anyone out there has more certain information, PLEASE COMMENT in the comment box beneath at least one of the photos or the video.