The driver accelerated. His pointer moved round a dial, and the train answered his small movements, gathering speed and noise. I was conscious only of being in the grip of a tremendous force that was hurling us over those three gleaming rails. We took the tunnel at a good thirty-five miles an hour, and the noise we made changed to a hollow roar! I could feel the train swerve and rock slightly as we rounded a curve; but I could see nothing save here and there a green light close to the ground. If you can imagine that you are tied to a projectile shot from a gun in the night, you have an idea of driving an electric train through a tunnel. In the underground blackness stations show first as a faint yellow glow cut across by the jet-black semicircle of the tunnel. The next second you can see their curving rows of lights; they straighten out, and then the platform at which you will pull up lies level as a knife edge before you. Mark Lane ... Mansion House ... Blackfriars ... Temple ... Charing Cross. Charing Cross is big. As you sweep in the driver has time to collect a lightning series of snapshots! A bookstall, a cigarette booth, lit and yellow, a pretty girl coming down the steps carrying a bag, a fussy old lady asking a ticket inspector how to get to Baron's Court, and a sudden stir and interest of Ealing-bound people who detach themselves from the crowd of waiting passengers. Just a flash! All seen in the fraction of a second! Bells ring down the train. A loud one clangs in the engine cab. And off you go again through the blackness towards Victoria. Few things are more uncanny in mechanical London than the system of automatic signalling which permits a chain of electric trains to move over the same line at minute intervals with no chance of a collision. Little green lights beckon you on, telling you the way is clear. As you pass them red lights at your back change to green, beckoning on the train behind; and so it is all the way along. Now and then you meet a red light. You stop! The light changes to green. On you go! The marvellous thing is that if, in a moment of colour blindness, you tried to override a red light your train would correct you. It would refuse to go on! At Kensington we shot out into the open air. Gaily, madly, we raced over the shining rails, marvellously, so it seemed to me, taking a sharp bend, smoothly continuing along the straight. It was like flying without the perpetual anxiety of flight. Once, with the awful insolence of the cocksure, I thought the driver had erred. "Good Lord!" I said. "That was a red light, and _you've gone on_!" Instead of kicking me out on the metals, as he should have done, he smiled and remarked: "Wrong signal! That red light governs the loop line!"