Allow the westbound train to be "w" and the eastbound train to be "e". We can make the following equation:
4 (e + (e - 12)) = 680
4 (2e + 12) = 680
8e - 48 = 680
8e = 728
e = 91 mph
e - 12 = w
91 - 12 = w
79 = w
Therefore, the westbound train travels at 79 miles per hour.