Complex sentence: The structure and track is comprised mostly of 850,000 feet of Douglas Fir which was treated with a pesticide and is not considered environmentally friendly and is slowly being replaced with southern yellow pine.
Split into simpler sentences: The structure and track is comprised mostly of 850,000 feet of Douglas Fir which was treated with a pesticide. Thw wood is not considered environmentally friendly and is slowly being replaced with southern yellow pine.