A、Because they were driven by steam power. B、Because they did the work that animals used to do. C、Because they pulled cars full

In the early days of the railroads, horses pulled the trains. The trains had no power of their own. Richard Trevithik of England invented a steam-powered engine in 1804. Soon people were building railroads and steam engines all over the world. Because the steam engines did the work that animals used to do, people called them "iron horses".
    Peter Cooper was a rich American businessman. He owned a lot of land near the Baltimore and Ohio Railroad. He wanted the railroad to be successful. He built his own steam engine to drive along the railroad.
    Most people traveled in coaches drawn by horses. A coach line challenged Peter Cooper to a race.
    The day of the race came. At first the horse was winning the race. Peter Cooper’s engine needed time to build up steam. He worked hard to make the train go faster. Soon he was catching up with the horse, he was going to win the race! Suddenly one of the parts of the engine broke. The train stopped. The horse rushed ahead. Peter Cooper lost the race.
    Of course, that was not the end of the story. By 1870, railroad extended all across the United States. The "iron horse" had become an important part of American life,