I’ve found some notes in an old book on Woodhead Tunnel, they may be of interest, so I’ll summarise. Apologies for only having good old imperial measurements. If you don't know what a bushell or a pole is, tough.
The north bore was used, because it was continuously stone lined, the south bore was not. However, there was a problem of soot – up to almost a foot thick. Water could not be used to clear it because of pollution (reservoirs nearby), so it was blasted off by air jets.
The soot on the floor was mixed with lime, cement and old ballast, and set to make a decent hard floor – very clever.
The old stone lining didn’t have any weep holes, and ground water pressure was causing it to bulge (a common problem in lined tunnels), so grout was forced in, to strengthen and fill voids.
Almost half of the lining was repointed – 33000 sq yards. There were 6 ventilation shafts, 1,3&4 were filled with stone, 5 was partially filled, the biggest one, no. 2 was repointed so that it may be used in the future for ventilation if more cables were installed.
The south bore is used for emergency access, and is connected by cross passages and steel doors.
I understand that the old signal box in the middle is still there – it was not used for too long because of the crappy conditions. It was certainly there in the 60's 'cos a colleague at work has seen it.
The cables heat up, so they are insulated with oil impregnated paper, in an oil filled pressured tube. This pressurisation had to be kept constant, even when being transported.
The cables are in a water filled trough, the water is pumped at a rate of 132 gallons a minute from Woodhead, to Dunford and back again. Heat exchangers at the Woodhead end dissipate the heat. The temperature rise is around 15 degrees centigrade at Dunford, and 35 degrees by the time it is back at Woodhead.
The cables were made by Pirelli (did they get a calendar?) There are 2 circuits, 3 cables (phases) each. There is a narrow gauge railway which presumably is used for maintenance, there is also cooling air fed through, especially to cool the 125 cable joints. It was activated in 1969 and cost £2.75 million.