Can you summarize for me in no more than three paragraphs the book, "The Little Engine That Could", by Watty Piper?

The Little Engine That Could is a children's book written in 1930 by Watty Piper.  The main purpose of the book is to help bring out in children a positive, "can-do attitude."  The key phrase in the book is, "I think I can." 

The main character is the "Little Engine".  The Little Engine is an undersized locomotive that is called upon to pull rail cars full of good things for the boys and girls that live on the other side of the mountain.  Several other locomotives, appearing newer, stronger or more mature than the Little Engine all make excuses for why they will not help take the train cars over the mountain.

The Little Engine gladly takes on the task when called upon.  When starting to pull the heavy rail cars, The Little Engine strains with all its might and begins to say, "I think I can, I think I can, I think I can."  When climbing the mountain, it doesn't look like the Little Engine has the power to get over the peak.  The Little Engine puffs, "I think I can, I think I can, I think I can."  Finally The Little Engine reaches the top, comes over the crest and on the way down says, "I though I could, I thought I could, I thought I could."