答案：B

解析：

When the speed of a train in miles per hour is multiplied by the travel time in hours, the distance traveled by the train is obtained. The distance that the first train travels is 60 miles per hour1 hour, which is 60 miles. Using the variable x for the travel time, in hours, of the second train, the distance that the second train travels is 30 miles per hourx hours, which is 30x miles. Since the two trains travel the same distance, 60 = 30x, which gives x = 2 hours, the travel time of the second train.