You can go to Hulunbeier from Datong by train. The railway station in Hulunbeier city is called Hailaer Railway Station. Just take the Train K274 to get to Hailaer. The trip needs about 35 hours. I suggest you choose the sleeper comparment, as the trip is long. In Hailaer, it's very easy to find the tour bus to get to Hulunbeier Grassland. It takes about one hours to get to the grassland.