80' up the tunnel there is a tripwire, DC 18 to spot, which when triggered causes a loaded mine cart which nearly fills the tunnel to come rumbling down the ramp. Character have 2 rounds to exit the tunnel or they take 3d6 points of damage as it slams into them, no save. It only takes a DC 12 Disable to disable the trip wire. Once the trap is sprung, the track can be rigged to stop the cart with a DC 15 Disable check.