There is no public access but here is an image of the 1,024 yard tunnel that used to serve Kemp Town station. The station closed in 1932 but the tunnel was used for freight until 1971. It’s the reason for that train coming out of the corner of the Gala bingo hall.  Apparently the tunnel was like an egg, then the bottom part was levelled off.  You can still smell the soot and carbon to this day.