Mark the beginnings of the curves on both ends of the track with a piece of tape. Do this on the inner portion of the track.
Measure each long side on the inside of the track between the pieces of tape.
Lay a string along each inner curved end of the track. Mark the end point of the measurements on the string. Then measure the string with a tape measure.
Add all of the measurements to find the perimeter. For example, if each long side is 300 feet long, add 300 plus 300 to get 600. If each curved end was 150 feet long, add 150 plus 150 to 600 to get a perimeter of 900 feet.