An early version goes as follows;
A little railroad engine was employed about a station yard for such work as it was built for, pulling a few cars on and off the switches. One morning it was waiting for the next call when a long train of freight-cars asked a large engine in the roundhouse to take it over the hill "I can't; that is too much a pull for me," said the great engine built for hard work. Then the train asked another engine, and another, only to hear excuses and be refused. In desperation, the train asked the little switch engine to draw it up the grade and down on the other side. "I think I can," puffed the little locomotive, and put itself in front of the great heavy train. As it went on the little engine kept bravely puffing faster and faster, "I think I can, I think I can, I think I can."
As it neared the top of the grade, which had so discouraged the larger engines, it went more slowly. However, it still kept saying, "I--think--I--can, I--think--I--can." It reached the top by drawing on bravery and then went on down the grade, congratulating itself by saying, "I thought I could, I thought I could."