The Erlian border crossing is the land connection between China and Mongolia, and the Trans-Mongolian Railway passes through it on its way between Beijing and Ulaanbaatar. Even if you are not crossing, the infrastructure of the border zone is interesting to observe: the freight lorries lined up for customs inspection, the Chinese and Mongolian merchants doing business in both languages, and the general atmosphere of a place where two very different economies and cultures meet at a gap in the fence.
If you are crossing by rail, the bogie exchange at Erlian station takes several hours and the carriages are formally sealed during the process — passengers cannot leave the train, but there is usually enough window-watching to pass the time. The road crossing (for private vehicles and buses) is more straightforward. Walking across the border is not permitted; the crossing is 10 kilometres north of town and requires a vehicle. Zamiin-Uud on the Mongolian side has basic accommodation if you arrive too late for onward connections.