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 For 40 years a passenger service operated, but this was withdrawn except for the occasional enthusiasts' railtour.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.

A proposal to reopen the line to passengers with two stations at Marchwood and Hythe and trains to Southampton Central was first made in 2009 and was incorporated into the plans for the Restoring Your Railway fund. The fund was cancelled in September 2024, resulting in the proposal for restoring passenger services being scrapped as a result.

Authorised in 1903, after some years of trying, the line was built under the Light Railways Act 1896 as the Totton, Hythe and Fawley Light Railway and opened on 20 July 1925. It begins at South West Main Line at Totton, west of Southampton, where Bournemouth-bound trains run parallel with the branch for one mile (1.6 km) before curving away to the south.

The passenger service served Marchwood, Hythe, and Fawley. Between Hythe and Fawley there was a Hardley Halt which opened for workmen in 1958 and closed in 1965. Operated by steam trains, then the Hampshire' diesel-electric multiple units, the service was withdrawn on 14 February 1966.

At first, traffic was light but they subsequently expanded when the then largest oil refinery in Britain opened at Fawley in the 1920s. The line became part of the British Railways (Southern Region) following nationalisation in 1948. The first station to be closed was Hardley Halt on 5 April 1965, followed by Marchwood, Hythe and Fawley on 14 February 1966. The line was then only used by freight from Marchwood Military Port and Fawley Refinery until 2016.

On 16 June 2009 the Association of Train Operating Companies announced it was looking into the reopening of the railway as far as Hythe, with a possibility of a further extension to Fawley if agreement could be reached with Esso, which owns the land where Fawley railway station once stood. The proposals were:

It was envisaged that the railway link could be built over a five to 10 year period at a cost of around £3 million. The service would be operated by the then franchisee South West Trains using diesel multiple units (DMUs). If the scheme delivered a sufficient financial return, there would be a future possibility of electrification. The service was planned to run half-hourly during peak times and hourly at other times.

On 8 November 2013 Councillor David Harrison of Totton South and Marchwood obtained a copy of the final GRIP 3 Study report and shared it via his website. In the report it was stated that the service would be half-hourly, using DMUs calling at all stations between Hythe and Southampton, including a new station to be called Hounsdown (once planned to be called Totton West). A new passing loop would have to be installed at Hounsdown to allow passing of freight and passenger trains. Other upgrades would include AWS/TPWS and signalling. For reasons of security at the oil refinery, Fawley station would not be reopened as part of the scheme. It has been pointed out that there are some possible drawbacks to this scheme. For example, if the Waterside line gets the green light, the subsidy from Hampshire County Council for the Southampton & Hythe ferry service would be likely to cease, and the local bus companies which operate in the area might be at risk of losing some of their subsidy. On 21 January 2014 Hampshire County Council decided to shelve the plans to reopen the line. The council's report came down against committing further funding for the scheme due to a perceived poor value for money business case, although it said the authority should review the position should local circumstances change.