Why is it a loop and not straight?
It's the grade, or how much the train gains elevation for a given distance. For example, say the train had to go up 100 meters of elevation. Now assume a 1km long rail that goes in a straight line to go up the 100m. The grade is 100m/1000m or 10% grade. Now if you take the same elevation gain (100m), and put a loop in the rail, that will lengthen the rail to say 2km. The grade is now 100m/2000m or 5%. You can see with the loop, the grade is less than if it were straight, 5% vs 10%, and the lower grade means it is not as steep for very long and heavy trains to not only go up but to go down. Saves on fuel, the need for more power and general wear and tear.

I visited the Tehachapi loop a year prior to this video and maintenance was being done on the tracks. I waited two hours and no trains passed by. This time, as soon as I pulled up, there was a train staged at the loop going down grade. I thought I would miss him as I just passed him coming down as I was driving down the highway. Luckily he stopped. As soon as I got to the overlook, a train was starting to come up grade. The result is in the video. It's like poetry in motion. It's worth mentioning the beautiful scenery just driving to the overlook and beyond, truly enjoyable. The audio was taken from another freight train that I recorded. That train was going much faster than the trains in this video, as you might be able to tell, but it works well enough for my liking. Hope you enjoy.