听力原文： Although I think the United States generally has an excellent system of transportation, I do not think that it does a good job of transporting people between cities that are only a few hundred miles apart. A person commuting between Detroit and Chicago, or between San Francisco and Los Angeles, the so-called 'strip cities', may spend only a relatively short time in the air, while spending several hours getting to and from the airport. This situation makes flying almost as time-consuming as driving. Moreover, airplanes use a lot of their fuel just getting into the air. [15]They simply are not fuel efficient on short trips. High speed trains may be an answer. One fairly new proposal for such a train is for something called a maglev, meaning a magnetically levitated train. [16] Maglevs will not actually ride on the tracks, but will fly above tracks that are magnetically activated. [17] This will save wear and tear on the tracks. These trains will be able to go faster than 150 miles an hour. At that speed, conventional trains have trouble staying on the tracks. [14] As you can see, maglevs offer exciting possibilities for the future. What is the main topic of the passage?