I'm assuming that the author meant an "Underground Railroad" by the Civil War meaning — as in helping slaves escape to freedom — rather than an "underground railroad" by the modern meaning — as in a subway train.
non American, English speaker here, while I am overall familiar with the American underground railroad/ anti slavery network/ Harriet Tubman (Thanks Beau), I took the post authors words at face value as this is not an exclusively American story.
I didn't consider that the 150 novel a year, post author was using that meaning because....... The post author has shown in my opinion they lack nuanced understanding of concepts and story building. Eg. The MC has challenged the slave norms regularly. They are involved in release and harbouring fugitives, they are supporting them in their revolutionary activities. With the way the story is going, a Harriet Tubman style underground railroad i think is a part of the story progression.
This type of nuance and story building I believe Runey is capable of, and has proven themselves so far.
Long Live Runey, may they have endless patience and emotional capacity to put up with consumers of a free novel that don't know they can just stop reading if they are not happy.