Patcham Tunnel

Patcham Tunnel (or Compulsory Tunnel) is a railway tunnel on the Brighton Main Line through the South Downs between Preston Park and Hassocks in East Sussex, England. It is 446 metres (488 yards) long. Its construction was neither necessitated by the local geography nor originally intended but, following the objections of a local landowner, the tunnel's creation was specifically stipulated by Parliament as a condition for the authorising the line. Accordingly, the London and Brighton Railway had their line directed through a purpose-built tunnel instead of a cutting. Patcham Tunnel was constructed between 1840 and 1841; the work was beset by a collapse part-way through. Entering service as intended, the tunnel has demonstrated a tendency to flood and has been repeatedly blamed for the sporadic cancellations of services on the Brighton Main Line.

Tunnel data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).