Higham and Strood Tunnel
The Higham and Strood tunnel is 2.25 miles long, and was the second longest canal tunnel built in the UK.
It was built between 1819 to 1824 for the Thames and Medway Canal. A single track railway was laid on the tow-path in 1845, and was soon doubled by infilling the canal. A 100 yard air vent was cut into the middle when the tunnel.
It was also the largest tunnel at 35 feet high from arch to canal bed, 21.5 feet wide at the water line, a further 5 feet wide at towpath level, and had water 8 feet) deep.These dimensions could accommodate a 61 tonne sailing barge with its mast lowered.
The tunnel was dug through the chalk using only hand tools and was considered an engineering wonder of its time.
'The tunnel is so straight, that a person placed at one end, may discern a small light entering at the other extremity [...] On the opening of the tunnel, a small steam passage boat was employed for the conveyance of passengers from Gravesend to Rochester, and vice versa; but as it was found to injure the towing-path of the tunnel, as well as the banks of the canal, it was discontinued. Foot passengers, however, still pass to and fro, though some caution is necessary, in order to avoid coming into contact with the horse, or horses, towing the barges.' 
From 1845 the newly built railway between Gravesend and Strood shared the tunnel with the canal, a single track resting partially on the tow-path and partially on wooden stakes in the water.
'The ride through the dreary tunnel with the dark waters of the canal beneath us, and an insecure chalk roof above our heads, enlivened as it is by occasional shrieks from the engine's vaporous lungs, and the unceasing rattle of the train, is apt to make one feel somewhat nervous; and the first glimpse of bright daylight that breaks upon us, relieves us from a natural anxiety as to the chances we run of being crushed by the fall of some twenty tons of chalk from above, or being precipitated into twenty feet of water beneath, with the doors of the carriages locked and no "Nautilus belt" around our waists and not even a child’s caul in our pocket. This relief is however temporary, for the light only breaks in through a gap in the tunnel, and some more experienced traveller informs us we are only half out of it. However, our journey is brought to a close without any accident: and we embark on the steamer that is to deposit us at Chatham. 
In 1846 the canal company sold the tunnel to the South Eastern Railway, which filled the canal and laid a double railway track over it. This was part of the North Kent Line. The canal towing contractor's home was converted into the ticket office for Higham railway station.
Sources of Information
- Tallis Directory 1839
- William Orr, 1847
-  Wikipedia