I think that there can be a variety of approaches to take with an introduction for this topic. One particular approach I would take is to outline the conditions that gave rise to the Underground Railroad. This might include discussion of how slavery was a reality for thousands of people of color in the 19th Century. After this, perhaps a brief discussion about the role of the "conductors" on the railroad might be appropriate. Your final phase of the introduction would be to briefly explain how Harriet Tubman was a conductor and how her importance to the Underground Railroad helped to define its success for many slaves.
Another introduction could simply to talk about how history has regarded Harriet Tubman. Naturally, this approach to an introduction would discuss the broad strokes and major ideas behind Tubman, the Underground Railroad, and the importance attached to it. I think that in this introduction, you would be "introducing" many of the elements that you will be delving into later on in the paper. Yet, I would caution you here that if you were to take this introduction approach, it is extremely important that you stick to broad ideas in an opening manner. Indepth analysis will come in the body of your paper, so the introduction is to offer the reader a mere glimpse of what will be presented in the course of the paper.