The Horn of Mortal Danger
Lawrence Leonard
 Simon "Widgie" and Jen Widgeon are innocently exploring the abandoned Highgate rail tunnel near their home when they discover a hidden gateway halfway along it. Through this they find their way to a little old-fashioned railway station. At first the tunnels seem deserted. As Jen wanders down the tunnel to explore, the Railwaymen emerge and capture Widgie. Jen in her turn discovers an underground canal, complete with a little steamboat, which the railway crosses by means of a retractable bridge. Men emerge from the boat and take her captive. It quickly becomes apparent that Widgie and Jen have become caught up in an entire underground civilisation, the North London System, kept secret for centuries from the world above. Two civilisations, in fact, seemingly perpetually at war. Their arrival is the catalyst for a climactic battle between the Railway and the Canals. Widgie manages to escape and rescues Jen, and the realisation of the threat they pose should they escape above ground and expose the System forces the Railwaymen and the Canallers to set aside their differences for the time being in a vain effort to recapture them. In the course of the battle, giant Rats, kept imprisoned in a blocked-up tunnel, are released and proceed to spread through the whole system. As Widgie and Jen escape into the Post Office Railway, it appears that the entire civilisation is on the verge of disintegration under the assault from the Rats.