The Fawley branch line, also known as the Waterside line, is a standard-gauge railway line to Fawley, in the English county of Hampshire. It is on the opposite side of Southampton Water from the city of Southampton itself, in an area known as Waterside. For 40 years a passenger service operated, but this was withdrawn except for the occasional enthusiasts' railtour. The line serves the freight needs of Marchwood Military Port, having also served the same function for Fawley Refinery until 2016.

Marchwood Military Port (MMP) or Marchwood Sea Mounting Centre (SMC) is a military port located in Marchwood, Southampton on the south coast of the UK, and the base of 17 Port & Maritime Regiment Royal Logistic Corps. The port was built in 1943 to aid in the D-Day assault on Normandy in 1944 and has since been used to support the Falklands War. The port is now used largely by the Royal Fleet Auxiliary, as a base for their ships, including the Tide-class tankers, and it is also still employed for military cargo and personnel movement.