railroad tunnel(n = noun.artifact) - a tunnel through which the railroad track runs;
is a kind of tunnel
has particulars: channel tunnel, chunnel

carpal tunnel, carpal tunnel syndrome, channel tunnel, elevated railroad, railroad, railroad bed, railroad car, railroad engineer, railroad flat, railroad line, railroad man, railroad siding, railroad station, railroad terminal, railroad ticket, railroad tie, railroad track, railroad train, railroad vine, railroad worm, tunnel, tunnel stern, tunnel vision, underground railroad, wind tunnel