The Answer

Would you believe it's a chimney?

In the days of steam trains suffocating smoke in the tunnels meant that shafts had to be bored from the surface to ventilate the tunnel and the underground route could be followed by the line of large chimneys between the tunnel entrances. You can still follow the tunnel on google earth by spotting the chimneys. The trains are electric these days making the chimneys obsolete.

Here is a link to an interesting article about this very unusual tunnel entrance. It mentions the chimneys as well as the individual who lives in the cottage above the entrance.