I told you I have a friend, PDS, who knows everything*, and to prove it, read on. Rick alerted him to my post about tunnel rain and in due course we have this erudite reply.
A tricky question.
If condensation, then whether it rains on you depends on the morphology of the tunnel roof lining. If the roof were smooth, like a glazed clay pipe, the water would just run down the sides. If stone with mortar joints, then water will drip off every joint. Has Neil made a study of canal tunnel roofs as he 'legs' through them?
Crick tunnel is in the Lower Lias, which is normally thought of as clay. However there are plenty of beds of permeable material in it, so Neil's idea that water could permeate through certain layers is entirely sound. It would worry me if there was much water flow. The water could scour out clay and other loose material from just above the tunnel lining and lead to a collapse.
The Lias typically contains iron pyrites, FeS2, which oxidises on exposure to atmospheric oxygen to give the brown/red colour of iron oxide and SO2 (which would normally dissolve in the water). If oxygenated water passes through the Lias you could get acid water akin to acid mine drainage. Then the lime mortar in the tunnel lining could be dissolved.
It all sounds rather hazardous. I would advise Neil to avoid the tunnels. He should study the maps to find convenient bypasses! Alternatively he could 'leg' really fast so as to minimise his exposure to risk.
*Well nearly everything, although I suspect that his references to legging may be ironic.