Pierre, just to mention that the canal tunnel is on the western side of the Medway starting immediately after the curve northwards from Strood station and housing the railway to Gravesend

There are excellent aerial views of the tunnel (well as good a view of a tunnel as you can get from the air!) - including the short stretch of open line in the middle - on Windows Live Search maps, Birds Eye view

Where the railway turns south the canal went straight on into the Medway and it looks like there is still a hint of where the the two joined