On The Run (Obliterating the Deep State, #6)

After the untimely death of a beloved friend and colleague, things are looking bleak for Darren and his fellow defectors. Chased by bounty hunters on the surface and haunted by monsters lurking in the tunnels below, there doesn't seem to be any sanctuary that can be found. The world has gone mad, and even man-of-steel Darren Mathews is beginning to lose his resolve.

There is an underground train, which was previously discovered, but it goes somewhere that their group is unfamiliar with. Are ...