The underground railroad was not a railroad. It was a codename for a loose network of people who helped runaway slaves escape from the South to the North, (or to
Canada, after the
Fugitive Slave Acts). This was not an official organization that one could join. Some people on the railroad just gave someone a lift in the back of their wagon every once in a while and didn't say anything when the
bounty hunters showed up. Others spent serious time and money building
secret rooms and underground chambers into their houses which could be used to hide runaways.
There were "conductors" on the underground railroad - people who acted as guides to lead people on the treacherous journey North. Harriet Tubman is probably the most famous of these. There were codes that could be used to find safehouses and transportation. (like the quilt code, for example, or numerous songs that were used to transmit information)
In spite of all the historical attention given to the underground railroad, most escaping slaves barely used it. The fact that it existed and that the codes to find it were as widespread as they were is incredible, especially considering the deep secrecy surrounding it and the isolation of most escaping slaves. The fact that such a network could survive at all in such a hostile environment is something of a miracle.