When you next visit a tunnel, think in four dimensions. Don't just examine its fabric - wander back to a time before it existed.
In your mind's eye, picture the scene as men descended on the hillside to set up their encampments: accommodation for humans, stables for horses, magazines for explosives. Over many months - maybe several years - they overcame all that nature could throw at them to drive a conduit for trains. Some wouldn't see the project come to its fruition; those who did would reap meagre financial rewards. And the passengers it benefited would pass through in seconds without giving the darkness a second glance.
But these were awesome endeavours. How did 19th century engineers create them without the mechanical and technological advantages of the 21st century?