Know Before You Go
The least strenuous hike to see the tunnel involves parking along side the modern rail track crossing at the bottom of the hill and hiking in along the mostly flat, but muddy and wet and somewhat overgrown eastern approach. There is a small area where a couple of cars can park at the crossing.
Hiking in will be very muddy and wet most times of the year so wear some mud boots. Bring a flashlight. The hike is only about 1/4 of a mile long and is pretty obvious given the terrain and steep hills on either side.
If you have a 4x4, and don't mind getting it scratched up, there are a couple of old trails at the top of the hill that lead down to the western side of the tunnel. Tire tracks indicate that people have driven down to the Western entrance and through the tunnel before. This is not generally recommended as current access to the tunnel is dependent on being discrete and not disturbing the neighbors that live in the area.