Waterloo station and Clapham Junction, London.

L/S of railway tracks from the point of view of the train leaving Waterloo station, the train moves forward on its journey, there are other trains passing in the opposite direction. M/S of the driver in the cab steering the train. The train passes a signal. L/S of the tracks shot from the front of the train, C/U of driver's profile, the train passes under another set of signals. C/U of the driver's hands on the controls.

M/S of the tracks from train point of view, the camera moves up to a signal box which says 'Clapham Junction', the train passes it. C/U of driver's profile, the train passes under another set of signals.

M/S of the interior of the Southern Regions Signals School with three men looking at all the model frame and block signals, one of the men reaches behind him to do something off screen. C/U of him reaching over and then turning back and pulling a lever down. C/U of an electric signalling panel with hands in the shot, operating it. C/U of three dials. C/U of a model signal which goes up and a model train which comes out of a shed, the signal drops again.

M/S of the three men at the controls with the model train going past in the foreground. M/S of dials in a row. M/S of one of the men pulling down a lever to change a signal. The model train starts up again and drives past the signal. M/S of the train driving along the track in front of the signal panels and the three men.

M/S of Acting Inspector Allen of Southampton and a pupil at the red staff container, the pupil is tapping a bell. C/U of the steam engine on the model train.

M/S of a pupil taking a staff from the container. L/S of the pupil giving the staff to Inspector Allen, there is a third man on the left watching. Inspector Allen gives the pupil a model staff in return, which he places in the cab. He stands back and rings the bell on the container and the train moves off.

M/S of points changing on the model track. M/S of the three men, the pupil rings the bell again and pulls a lever on a miniature frame to represent mechanical locking. Inspector Allen takes the top off the unit to reveal the levers, C/U of the levers being operated. C/U of the mechanics of the signal and a demonstration of how it is impossible to trip a conflicting signal. C/U of the pupil's face looking very serious.

M/S of a model signal going up, the camera moves down and shows the train running along underneath on the model tracks, the signal drops when it has gone through.