In addition to Sword's suggestion, there is a patch of intense radiation inside the tunnel, and it could be that you walked into that. Look for a message appearing at the top of your HUD that says "RADIATION DANGER", and open your inventory to see if the yellow radiation bar is full.
The area of radiation is in the centre of the tracks about half way down the tunnel. The best way to avoid it is to pass the engine on the right-hand side and stick to the tunnel wall on the right until you are near the far end.