Harwood Train Yard
There is very little known history of the Harwood Train Yard. All that's known, is that the yard was part of a railroad line connecting, Portland View and Portland Harbor via a tunnel underneath the district of Saint Mark's. No one knows exactly when the line was abandoned, but the line appears as though it hasn't been used in over 50 years, at the most.
Today, (as of 2001 in GTA:III) the yard still exsists with track and a few old rusting away flat cars. The tunnel, is still there to this day, but most of the rails have been removed in it. (Only a tiny section goes into the tunnel before it ends abruptly.) Probably the line belonged to the Brown Streak Railroad at some point, before the events of Grand Theft Auto: San Andreas. The player is able to enter the tunnel. To do this you must, drive around the old loading freight platforms, and then you're on the tracks. Then turn west and you're officially in the tunnel, and you can exit near a store in the neighborhood of Portland Harbor.