As it was later discovered a thrust fault caused by subterranean pressure buckled one layer of rock over another over thousands of years creating a nightmarish "porridge stone" or demoralized rock. This inherently unstable stone slowed progress to a crawl and forced the creation of a tube of brick (which was essentially a brick arch with a supporting invert as a base) of 883 feet. When the tunnel was completed the brick lining extended 7573 feet in from the West Portal. Later in the 1972 a metal lining was installed after a section of the brick arching collapsed several hundred feet inside the tunnel. A 318 foot deep shaft known as West Shaft was sunk 2500 feet from present day West Portal so the weak stone could be circumvented and work could continue from the inside to outside. (In other words 2 headings were added, one going into the mountain, one going towards West portal).
One of The West Portal's hidden secrets is known as the Haupt Tunnel, or Hoosac's baby brother. This abandoned section was used for housing mules, as well as drainage after the main approach was complete. It was never actually part of the active train tunnel. Think of the the current situation with the West Portal. There is a long approach cut into the mountain before you actually get to the portal itself. Well, during the original construction a similar approach was cut about 100 feet south of today's cut. This cut leads up to an old no longer existing West Portal that is in line with the present tunnel and about 300 feet west. This cut can still be seen today, it leads up to the pond from the east near the parking area and follows to the right (south) of the 4 wheeler trail. This cut often fills with water when there is a high enough water level. This cut at present day suddenly ends. There isn't much to see.
The interesting part lays underneath the ground where the original 300 feet of the Hoosac Tunnel remains. If you walk in the cut with the active railroad towards the portal about 150 feet west of the portal you will notice a large amount of boulders on the south side of the cut. You will also notice a little trail leading up to these boulders. There is a small little crawl hole between some of the boulders. This is the entrance to the abandoned portion of the Hoosac Tunnel! It is about 150 feet long, underground, and has an abundance of cave ins (see pictures below). It is poorly ventilated, damp, musty and cold year round. I don't suggest anyone go in here, but obviously curiosity will get the best of anyone. Wear a hard hat and take a flashlight and remember that most of the fresh air in there comes out of that tiny hole you used to get in!