Why are the high-speed rail tracks from Beijing to Chifeng built in caves?

Because there are many mountains. High-speed rail is a combination of multiple trains, unlike cars that can crawl through as long as there is a road on the mountain. The high-speed rail only goes straight, and when it meets a mountain, it will pass through a cave. The railway from Beijing to Chifeng has a mileage of 503 kilometers, and the train running time is 1 16.7 minutes.