About the only likely situation would be one in which a previously double-tracked line has had one track removed, something that happens fairly often. I can imagine circumstances under which the tunnel would be left with both tracks, but they would be uncommon ones, such as climactic extremes that make covered trackage desirable. Or the track in the tunnel might be one end of a siding that needs to be in that specific location for geographic reasons, with the rest of the second track removed.
Mark Alan Miller