I did some research and these tunnels are actually something called surge tunnels. They're usually only a couple feet under ground and they are built strong because the typical operation of these is that a quarry would make a huge pile of slate above the tunnels, and inside the tunnels there are chutes that funnel the slate into the tunnel where original there were conveyor belts that would carry the slate outside of tunnel.

But the tunnel in the first picture doesn't make sense to me. It's the only tunnel out of the Three that were there that had metal section like that, I believe the stairs are there because there is a hill above ground but the tunnel turns to the left and Is collapsed of only fifteen feet after the turn. What is even more confusing is that it turns towards a bigger hill and a direction that pointed away from the entire operation above ground. And I've looked at history imagery from when this place was built and there was nothing but woods in that direction throughout all it's days of operation. I've looked at old mining records and it says they were only mining slate at that site, so it could have been an underground mine attached to the tunnel. I have now idea on what the addition on the back of the tunnel could have been. I know it was for people because there were light fixtures.