The famous Pequabuck Tunnel. Built 1910, 3500+ feet length. It allowed trains a direct route under Sylvan Hill, rather than the torturous switch-back climb that was used for the first 50+ years.

As was routine back in the day, several men died from cave-ins, and the tunnel is said to be haunted by ghostly apparitions with shovels. I myself cannot speak to that one way or the other as my bravery seems to wane quickly somewhere near the 150 foot mark.