Read the passage.
Secrets of the Underground Railroad
In the decades before the Civil War, thousands of enslaved Americans fled captivity via the Underground Railroad, a secret network of escape routes. Volunteers provided navigation, provisions, and hiding places for the fugitives—highly dangerous work for all involved. Those who were fleeing slavery could be captured and returned to bondage, while those who helped them could be jailed. The organizers reduced the risk by using code words to communicate sensitive information. For example, guides were referred to as "conductors," safe houses were called "stations," and the people who hid the formerly enslaved were "stationmasters." The formerly enslaved people themselves were referred to as "cargo," "packages," or "goods" in order to preserve their safety. Consider this text from a letter sent between two volunteers: "I suppose you are somewhat uneasy because the goods did not come safe to hand on Monday evening . . . The train only was from Harrisburg to Reading, and as it happened, the goods had to stay all night with us."
What is the main, or central, idea of the passage?