Why did slaves make the underground railroad?

Answer 1

I think you've interpreted the term a bit too literally. It wasn't a real railroad, it wasn't underground, and it wasn't built by slaves. It was a network of escape routes organized by anti-slavery activists, including both whites and free blacks, in the years leading up to the Civil War.

Escaped slaves would be moved from one "safe house" to another, often at night, so that they could eventually make their way to a northern state where they would be free and out of range of southern slave hunters. Because the activity was designed to be out of sight as much as possible and so many escapees were being rescued, the popular press compared it to a railroad that was running underground.

In fact, to look at it from the perspective of someone who knows about real railroads, at that time the idea of an actual underground railroad was a complete fantasy - the technology didn't even exist until the 1860s. But the slave escape routes were sufficiently successful that the public decided only such a science-fiction concept could be capable of moving so many people with such stealth.