Not sure what you are saying. A tornado won't hit the train, which is underground. If the tornado causes power outages that prevent the train from running, you could still run buses. Even if in a freakish set of circumstances it took out the train's power source AND destroyed the entire DIA bus fleet, you could still easily bring in buses from outside the airport to move people to/from the terminals (though the airport would likely be closed if the destruction was that bad, meaning you'd only have to remove people in the terminals, not bus new pax out.)