The Underground Railroad

I'm re-watching the first series and just finished the scene where Constance asks George for permission to run a station on the underground railroad (it's just before George foolishly agrees to take Virgilia with the family to South Carolina). George states how dangerous it is and warns her that she could face prison time "at least". As they are walking back to the house he stresses that no one must find out. The camera then cuts away to Stanley's wife looking down on them with a devious smile on her face. It's not clear that she knows exactly what's going on, but the scene plays out as a set-up for some future scene where Stanley and Isabel try to blackmail Constance and George over the information. But unless I'm mistaken, the underground railroad plotline is never really brought up again. Is this something that's explored more in the book?