The windows, all dim with the yellow breath of the tunnel, rattle down into their places, and you see once more the dip and catch of the telegraph wires beside the line, and the straight-cut hawthorn hedges with the tiny baby trees growing up out of them every thirty yards.
All this, of course, is what a tunnel means when you are in a train.

But everything is quite different when you walk into a tunnel on your own feet, and tread on shifting, sliding stones and gravel on a path that curves downwards from the shining metals to the wall.

Then you see slimy, oozy trickles of water running down the inside of the tunnel, and you notice that the bricks are not red or brown, as they are at the tunnel's mouth, but dull, sticky, sickly green.

Your voice, when you speak, is quite changed from what it was out in the sunshine, and it is a long time before the tunnel is quite dark.
It was not yet quite dark in the tunnel when Phyllis caught at Bobbie's skirt, ripping out half a yard of gathers, but no one noticed this at the time.
"I want to go back," she said, "I don't like it.

It'll be pitch dark in a minute.