Here is the description by the person who posted the video on Youtube:
"This is a short clip from the Summer of 1952 of the West side of the Alpine Tunnel (Colorado). Now it is the Historic Alpine tunnel. In 1952 it was just the Alpine tunnel. Note that many ties are still in place even though the rails are long gone. The first frame in the clip shows the top of the tunnel opening. This movie was taken 42 years after the last train went through the tunnel. The mountain was already in the process of reclaiming itself as shown by the damage caused by falling rock. Today, both ends of the tunnel are blocked by rock slides. Mother nature always has her way in the end.
Freeze the first two frames in the movie to see the top of the tunnel opening West end in 1952,
There is a video of the inside of the tunnel from 1992 that is very interesting that shows the the tunnel explorers going in at this point I don't think anyone has been in there since 1992 as it is now sealed from both ends. Restoration would be a huge undertaking however so far, no one has come forward to fund such a project."