the underground railroad

     The "underground railroad" does not mean the subway, although it seems a logical error. The underground railroad refers to the chain of "safe" houses that once made a "network" through which slaves from the American South could escape to the North--where they would be free. There are still a few old houses that have hidden cubbyholes--like closets--where a runaway slave might have been hidden, in case anyone had followed him or her. The federal law required people to return runaway slaves, but this law was impossible to enforce in the North. People had to be careful anyway, and would hide the runaway slave if strangers approached.