The longest tunnel in the UK was supposed to be the London Underground Northern Line tunnel, which runs from the north of London, south, under the Thames and emerges in south London.
On Liverpool's Merseyrail: Since the 1970s when new tunnels were constructed in Liverpool's centre, from Birkenhead Park station to Brunswick Station at Liverpool's South Docks is all one tunnel with only air shafts in strategic places. Does anyone know the total length of this tunnel run? It must be approaching 5 miles. It must be one of the longest, if not the longest tunnel lengths in the country.