They went on, as fast as they could with caution. Crawling up the ridge, they peered over. Nothing was to be seen in either direction. They crawled down the other slope, and came to what appeared to be a sunken grass road. It was shadowed by the ridge. Looking to right and left, and discovering nothing, they got up and began to walk across the road. Suddenly Harry stumbled, and uttered a low exclamation. "A whack on the toe," he murmured. "By George!" whispered Kenneth behind him. He had stooped to look at the obstruction. Harry turned. The obstacle was a rail. There was no glint from it; apparently it was rusty. But it was sticky to the touch. Kenneth held his fingers to his nose. They smelt of tar. Beside the rail there was a layer of loose grass, twigs, rubbish of all sorts, and beyond this, five feet away, a parallel rail. "We have come on a single-track railway," said Kenneth. "It’s not marked on the map; must have been recently laid. Let us go on a little, and examine it." In a few minutes their discovery was confirmed. The seeming grass road was a roughly laid track. But the rails had been painted over with tar, and the sleepers and permanent way were hidden under low heaps of litter. "They’re clever beasts," said Kenneth. "D’you see the trick? No airman would ever guess this to be a railway. The rails are quite dark." "But what’s it for?" At this moment came the report of the gun, some distance to the south. "That’s what we are going to find out," said Kenneth. They made their way stealthily along the track between the rails in the direction of the sound. Presently, at a gentle curve, they came to a white post with a small square platform in front of it, abutting on the railway. Wondering what it was for, they went on, and in a few moments heard the rumble of an approaching train. They scrambled up the ridge on their right, threw themselves flat on the ground and watched. In a few minutes an engine and two trucks glided into view, making extraordinarily little noise. They passed slowly below the watchers. There was no smoke from the engine; perhaps it was electric. The first truck carried a heavy gun; the other, containing men, was like an ordinary railway wagon, but apparently better sprung, for it moved with only the low rumble which the watchers had already heard. The effect of the train gliding past, dark, almost without sound, was mysteriously strange.