There is, I believe, a tunnel which runs under, or near to this platform.

There are 2 tunnels.

The first is an old railway tunnel which runs diagonally from the north west side to the south east side under the majority of the platforms. Originally the station had its small Goods Yard on the west side, but as the town and traffic expanded it was found to be too small. A larger Goods Yard was built on the south east side, but on lower land. Due to the lower level of land on the east side the tunnel was built under the existing platforms. The "goods" line tunnel was used in WWII for air raids and housing "protected" important services to enable keeping the station running during this difficult time. Additionally I also heard that the tunnel was used for shooting practice at this time as well.

The second tunnel, a much smaller one was for moving goods using wagons, from the lower Goods Yard up to the station level proper, to avoid the steep incline of Trafalgar Street. This went from Trafalgar Street (just below the old toilets) northwards (in the same alignments as the platforms) and then turned on itself through 180degs, going southwards, but at a slight incline making it easier for the horses. It runs under the platforms on the Lewes side, and it can be noted that these platforms are "suspended" wooden platforms, which span the old tunnel.

The Station Tour during the Brighton Festival, by Jackie Marsh-Hobbs, is well worthwhile if anyone wants to visit these old tunnels, as well as finding out much more of the history of the station.